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<title>NuttX Users Manual</title>
<meta name="AUTHOR" content="Gregory Nutt">
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<table width ="100%">
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    <td>
      <h1><big><font color="#3c34ec"><i>NuttX Operating System<p>User's Manual</i></font></big></h1>
      <p><small>by</small></p>
      <p>Gregory Nutt<p>
      <p>Last Updated: October 16, 2019</p>
    </td>
  </tr>
</table>
<hr><hr>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Introduction"><h1>1.0 Introduction</h1></a>
  </td>
  </tr>
</table>

<p>
  This manual provides general usage information for the NuttX RTOS from the
  perspective of the firmware developer.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="overview"><h2>1.1 Document Overview</h2></a>
  </td>
  </tr>
</table>

<p>
  This user's manual is divided into three sections plus a index:
</p>
<ul>
  <li>
    <b>Section 1.0, <a href="#Introduction">Introduction</a></b>:
    This section provides an overview of the NuttX user's manual.
  </li>
  <li>
    <b>Section 2.0, <a href="#OS_Interfaces">OS Interfaces</a></b>:
    This section details the program interfaces provided by NuttX.
    This section is divided into several paragraphs that describe different groups of OS interfaces:
    <ul>
      <li>Paragraph 2.1 <a href="#Task_Control">Task Control Interfaces</a></li>
      <li>Paragraph 2.2 <a href="#Task_Schedule">Task Scheduling Interfaces</a></li>
      <li>Paragraph 2.3 <a href="#Task_Switch">Task Control Interfaces</a></li>
      <li>Paragraph 2.4 <a href="#Message_Queue">Named Message Queue Interfaces</a></li>
      <li>Paragraph 2.5 <a href="#Semaphores">Counting Semaphore Interfaces</a></li>
      <li>Paragraph 2.6 <a href="#ClocksNTimers">Clocks and Timers</a></li>
      <li>Paragraph 2.7 <a href="#Signals">Signal Interfaces</a></li>
      <li>Paragraph 2.8 <a href="#Pthread">Pthread Interfaces</a></li>
      <li>Paragraph 2.9 <a href="#Environ">Environment Variables</a></li>
      <li>Paragraph 2.10 <a href="#FileSystem">File System Interfaces</a></li>
      <li>Paragraph 2.11 <a href="#Network">Network Interfaces</a></li>
      <li>Paragraph 2.12 <a href="#SharedMemory">Shared Memory Interfaces</a></li>
    </ul>
  </li>
  <li>
    <b>Section 3.0, <a href="#Data_Structures">OS Data Structures</a></b>:
    This section documents the data structures that are used at the NuttX
    interface.
    <ul>
      <li>Paragraph 3.1 <a href="#ScalarType">Scalar Types</a></li>
      <li>Paragraph 3.2 <a href="#HiddenStructures">Hidden Interface Structures</a></li>
      <li>Paragraph 3.3 <a href="#ErrnoAccess">Access to the <code>errno</code> Variable</a></li>
      <li>Paragraph 3.4 <a href="#UserStructures">User Interface Structures</a></li>
    </ul>
  </li>
  <li>
    <a href="#index"><b>Index</b></a>
  </li>
</ul>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="scope"><h2>1.2 Intended Audience and Scope</h2></a>
  </td>
  </tr>
</table>

<p>
  The intended audience for this document are firmware developers who are implementing applications on NuttX.
  Specifically, this documented is limited to addressing only NuttX RTOS APIs that are available to the application developer.
  As such, this document does not focus on any technical details of the organization or implementation of NuttX.
  Those technical details are provided in the <a href="NuttxPortingGuide.html">NuttX Porting Guide</a>.
</p>
<p>
  Information about configuring and building NuttX is also needed by the application developer.
  That information can also be found in the <a href="NuttxPortingGuide.html#configandbuild">NuttX Porting Guide</a>.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="OS_Interfaces"><h1>2.0 OS Interfaces</h1></a>
  </td>
  </tr>
</table>

<p>
  This section describes each C-callable interface to the NuttX
  Operating System. The description of each interface is presented
  in the following format:
<p>
<b>Function Prototype:</b> The C prototype of the interface function
is provided.
<p>
<b>Description:</b> The operation performed by the interface function
is discussed.
<p>
<b>Input Parameters:</b> All input parameters are listed along
with brief descriptions of each input parameter.
<p>
<b>Returned Value:</b> All possible values returned by the interface
function are listed. Values returned as side-effects (through
pointer input parameters or through global variables) will be
addressed in the description of the interface function.
<p>
<b>Assumptions/Limitations:</b> Any unusual assumptions made by
the interface function or any non-obvious limitations to the use
of the interface function will be indicated here.
<p>
<b>POSIX Compatibility:</b> Any significant differences between the
NuttX interface and its corresponding POSIX interface will be noted
here.
<p>
NOTE: In order to achieve an independent name space for the NuttX
interface functions, differences in function names and types are
to be expected and will not be identified as differences in these
paragraphs.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Task_Control"><h2>2.1 Task Control Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  <b>Tasks</b>.
  NuttX is a flat address OS.  As such it does not support <i>processes</i>
  in the way that, say, Linux does.
  NuttX only supports simple threads running within the same address space.
  However, the programming model makes a distinction between <i>tasks</i>
  and <i>pthreads</i>:
</p>
<ul>
  <li><i>tasks</i> are threads which have a degree of independence
  <li><a href="#Pthread"><i>pthreads</i></a> share some resources.
</ul>
<p>
  <b>File Descriptors and Streams</b>.
  This applies, in particular, in the area of opened file descriptors and streams.
  When a task is started using the interfaces in this section, it will be created
  with at most three open files.
</p>
</p>
  If CONFIG_DEV_CONSOLE is defined, the first three file descriptors (corresponding
  to stdin, stdout, stderr) will be duplicated for the new task.
  Since these file descriptors are duplicated, the child task can free close
  them or manipulate them in any way without effecting the parent task.
  File-related operations (open, close, etc.) within a task will have no effect
  on other tasks.
  Since the three file descriptors are duplicated, it is also possible to perform
  some level of redirection.
</p>
<p>
  pthreads, on the other hand, will always share file descriptors with the parent
  thread.  In this case, file operations will have effect only all pthreads the
  were started from the same parent thread.
</p>
<p><b>Executing Programs within a File System</b>.
  NuttX also provides internal interfaces for the execution of separately built
  programs that reside in a file system.
  These internal interfaces are, however, non-standard and are documented with the
  NuttX <a href="NuttXBinfmt.html">binary loader</a> and
  <a href="NuttXNxFlat.html#binfmt">NXFLAT</a>.
</p>
<p><b>Task Control Interfaces</b>.
  The following task control interfaces are provided by NuttX:
</p>
<p>
  Non-standard task control interfaces inspired by VxWorks interfaces:
</p>
<ul>
  <li><a href="#taskcreate">2.1.1 task_create</a></li>
  <li><a href="#taskinit">2.1.2 task_init</a></li>
  <li><a href="#taskactivate">2.1.3 task_activate</a></li>
  <li><a href="#taskdelete">2.1.4 task_delete</a></li>
  <li><a href="#taskrestart">2.1.5 task_restart</a></li>
</ul>
<p>
  Non-standard extensions to VxWorks-like interfaces to support POSIX <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:cancellation-points">Cancellation Points</a>.
</p>
<ul>
  <li><a href="#tasksetcancelstate">2.1.6 task_setcancelstate</a></li>
  <li><a href="#tasksetcanceltype">2.1.7 task_setcanceltype</a></li>
  <li><a href="#tasktestcancel">2.1.8 task_testcancel</a></li>
</ul>
<p>
  Standard interfaces
</p>
<ul>
  <li><a href="#exit">2.1.9 exit</a></li>
  <li><a href="#getpid">2.1.10 getpid</a></li>
</ul>
<p>
  Standard <code>vfork</code> and <code>exec[v|l]</code> interfaces:
</p>
<ul>
  <li><a href="#vfork">2.1.11 vfork</a></li>
  <li><a href="#exec">2.1.12 exec</a></li>
  <li><a href="#execv">2.1.13 execv</a></li>
  <li><a href="#execl">2.1.14 execl</a></li>
</ul>
<p>
  Standard <code>posix_spawn</code> interfaces:
</p>
<ul>
  <li><a href="#posix_spawn">2.1.14 posix_spawn and posix_spawnp</a></li>
  <li><a href="#posix_spawn_file_actions_init">2.1.15 posix_spawn_file_actions_init</a></li>
  <li><a href="#posix_spawn_file_actions_destroy">2.1.16 posix_spawn_file_actions_destroy</a></li>
  <li><a href="#posix_spawn_file_actions_addclose">2.1.17 posix_spawn_file_actions_addclose</a></li>
  <li><a href="#posix_spawn_file_actions_adddup2">2.1.18 posix_spawn_file_actions_adddup2</a></li>
  <li><a href="#posix_spawn_file_actions_addopen">2.1.19 posix_spawn_file_actions_addopen</a></li>
  <li><a href="#posix_spawnattr_init">2.1.20 posix_spawnattr_init</a></li>
  <li><a href="#posix_spawnattr_getflags">2.1.21 posix_spawnattr_getflags</a></li>
  <li><a href="#posix_spawnattr_getschedparam">2.1.22 posix_spawnattr_getschedparam</a></li>
  <li><a href="#posix_spawnattr_getschedpolicy">2.1.23 posix_spawnattr_getschedpolicy</a></li>
  <li><a href="#posix_spawnattr_getsigmask">2.1.24 posix_spawnattr_getsigmask</a></li>
  <li><a href="#posix_spawnattr_setflags">2.1.25 posix_spawnattr_setflags</a></li>
  <li><a href="#posix_spawnattr_setschedparam">2.1.26 posix_spawnattr_setschedparam</a></li>
  <li><a href="#posix_spawnattr_setschedpolicy">2.1.27 posix_spawnattr_setschedpolicy</a></li>
  <li><a href="#posix_spawnattr_setsigmask">2.1.28 posix_spawnattr_setsigmask</a></li>
</ul>
<p>
  Non-standard task control interfaces inspired by <code>posix_spawn</code>:
</p>
<ul>
  <li><a href="#task_spawn">2.1.29 task_spawn</a></li>
  <li><a href="#task_spawnattr_getstacksize">2.1.30 task_spawnattr_getstacksize</a></li>
  <li><a href="#task_spawnattr_setstacksize">2.1.31 task_spawnattr_setstacksize</a></li>
  <li><a href="#posix_spawn_file_actions_init">2.1.32 posix_spawn_file_actions_init</a></li>
</ul>

<H3><a name="taskcreate">2.1.1 task_create</a></H3>

<p>
<b>Function Prototype:</b>
<ul><pre>
#include &lt;sched.h&gt;
int task_create(char *name, int priority, int stack_size, main_t entry, char * const argv[]);
</pre></ul>

<p>
<b>Description:</b>
   This function creates and activates a new task with a
   specified priority and returns its system-assigned ID.
</p>

<p>The entry address entry is the address of the &quot;main&quot;
   function of the task.
   This function will be called once the C environment has been set up.
   The specified function will be called with four arguments.
   Should the specified routine return, a call to exit() will automatically be made.
</P>
<p>
   Note that an arbitrary number of arguments may be passed to the
   spawned functions.
</p>
<p>
   The arguments are copied (via <code>strdup</code>) so that the
   life of the passed strings is not dependent on the life of the
   caller to <code>task_create()</code>.
</p>
<p>
   The newly created task does not inherit scheduler characteristics
   from the parent task:  The new task is started at the
   default system priority and with the SCHED_FIFO scheduling
   policy.  These characteristics may be modified after the new
   task has been started.
</p>
<p>
   The newly created task does inherit the first three file
   descriptors (corresponding to stdin, stdout, and stderr) and
   redirection of standard I/O is supported.
</p>
<p>
<b>Input Parameters:</b>
  <ul>
    <li><code>name</code>. Name of the new task</LI>
    <li><code>priority</code>. Priority of the new task</LI>
    <li><code>stack_size</code>. size (in bytes) of the stack needed</LI>
    <li><code>entry</code>. Entry point of a new task</LI>
    <li><code>argv</code>. A pointer to an array of input parameters.
       The array should be terminated with a NULL argv[] value.
       If no parameters are required, argv may be NULL.
  </ul>
<p>
  <b>Returned Value:</b>
</P>
<ul>
<li>
  Returns the non-zero task ID of the new task or
  ERROR if memory is insufficient or the task cannot be
  created (<a href="#ErrnoAccess"><code>errno</code></a> is not set).
</LI>
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<pre>
   int taskSpawn(char *name, int priority, int options, int stackSize, FUNCPTR entryPt,
                 int arg1, int arg2, int arg3, int arg4, int arg5,
                 int arg6, int arg7, int arg8, int arg9, int arg10);
</pre>

<p>
  The NuttX task_create() differs from VxWorks' taskSpawn() in the
  following ways:
</p>
<ul>
<li>Interface name
<li>Various differences in types of arguments
<li>There is no options argument.
<li>A variable number of parameters can be passed to a task (VxWorks supports ten).
</ul>

<H3><a name="taskinit">2.1.2 task_init</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
   #include &lt;sched.h&gt;
   int task_init(struct tcb_s *tcb, char *name, int priority, uint32_t *stack, uint32_t stack_size,
                 maint_t entry, char * const argv[]);
</pre>

<p>
<b>Description:</b>
<p>
  This function initializes a Task Control Block (TCB)
  in preparation for starting a new thread.  It performs a subset
  of the functionality of <code>task_create()</code> (see above).
</P>
<p>
  Unlike task_create(), task_init() does not activate the task.
  This must be done by calling task_activate().
</P>
<p>
<b>Input Parameters:</b>
  <ul>
    <li><code>tcb</code>. Address of the new task's TCB
    <li><code>name</code>. Name of the new task (not used)
    <li><code>priority</code>. Priority of the new task
    <li><code>stack</code>. Start of the pre-allocated stack
    <li><code>stack_size</code>. size (in bytes) of the pre-allocated stack
    <li><code>entry</code>. Entry point of a new task
    <li><code>argv</code>. A pointer to an array of input parameters.
       The array should be terminated with a NULL argv[] value.
       If no parameters are required, argv may be NULL.
  </ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<ul>
  <li><p>OK, or ERROR if the task cannot be initialized.</P>
  <p>This function can only failure is it is unable to assign
    a new, unique task ID to the TCB (<a href="#ErrnoAccess"><code>errno</code></a> is not set).</P>
</ul>
<p>
<b>Assumptions/Limitations:</b>
<ul>
<li>task_init() is provided to support internal OS functionality.  It is
<b>not recommended</b> for normal usage.  task_create() is the preferred
mechanism to initialize and start a new task.
</ul>
<p>
<b>POSIX Compatibility:</b> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<pre>
   STATUS taskInit(WIND_TCB *pTcb, char *name, int priority, int options, uint32_t *pStackBase, int stackSize,
                   FUNCPTR entryPt, int arg1, int arg2, int arg3, int arg4, int arg5,
                   int arg6, int arg7, int arg8, int arg9, int arg10);
</pre>

<p>
  The NuttX task_init() differs from VxWorks' taskInit() in the
  following ways:
</p>
<ul>
<li>Interface name
<li>Various differences in types or arguments
<li>There is no options argument.
<li>A variable number of parameters can be passed to a task (VxWorks supports ten).
</ul>

<H3><a name="taskactivate">2.1.3 task_activate</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int task_activate(struct tcb_s *tcb);
</pre>

<p>
<b>Description:</b> This function activates tasks created by task_init().
Without activation, a task is ineligible for execution by the
scheduler.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>tcb</code>. The TCB for the task for the task (same as the
task_init argument).
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>OK, or ERROR if the task cannot be activated (<a href="#ErrnoAccess"><code>errno</code></a> is not set).
</ul>

<p>
<b>Assumptions/Limitations:</b>
<ul>
<li>task_activate() is provided to support internal OS functionality.  It is
<b>not recommended</b> for normal usage.  task_create() is the preferred
mechanism to initialize and start a new task.
</ul>
<p>
<b>POSIX Compatibility:</b> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<pre>
    STATUS taskActivate(int tid);
</pre>

<p>
  The NuttX task_activate() differs from VxWorks' taskActivate() in the
  following ways:
</p>
<ul>
<li>Function name
<li>With VxWork's taskActivate, the pid argument is supposed to be
the pointer to the WIND_TCB cast to an integer.
</ul>

<H3><a name="taskdelete">2.1.4 task_delete</a></H3>

<p>
<b>Function Prototype:</b>
<ul><pre>
#include &lt;sched.h&gt;
int task_delete(pid_t pid);
</pre></ul>

<p>
  <b>Description:</b>
  This function causes a specified task to cease to exist.
  Its  stack and TCB will be deallocated.
  This function is the companion to <code>task_create()</code>.
  This is the version of the function exposed to the user;
  it is simply a wrapper around the internal, <code>nxtask_terminate()</code> function.
</p>
<p>
  The logic in this function only deletes non-running tasks.
  If the <code>pid</code> parameter refers to the currently running task, then processing is redirected to <code>exit()</code>.
  This can only happen if a task calls <code>task_delete()</code> in order to delete itself.
</p>
<p>
  This function obeys the semantics of pthread cancellation:
  task deletion is deferred if cancellation is disabled or if deferred cancellation is supported (with <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:cancellation-points">Cancellation Points</a> enabled).
</p>
<p>
<b>Input Parameters:</b>
<ul>
  <li>
    <code>pid</code>.
    The task ID of the task to delete.
    An ID of zero signifies the calling task.
    Any attempt by the calling task will be automatically re-directed to <code>exit()</code>.
</ul>
<p>
<b>Returned Value:</b>
<ul>
  <li>
    <code>OK</code>, or <code>ERROR</code> if the task cannot be deleted.
    The <a href="#ErrnoAccess"><code>errno</code></a> is set to indicate the nature of the failure.
    This function can fail, for example, if the provided pid does not correspond to a currently executing task.
  </li>
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
  <code>task_delete()</code> must be used with caution:
  If the task holds resources (for example, allocated memory or semaphores needed by other tasks), then <code>task_delete()</code> can strand those resources.
</p>
<p>
<b>POSIX Compatibility:</b>
  This is a NON-POSIX interface.
  VxWorks provides the following similar interface:
</p>
<ul><pre>
STATUS taskDelete(int tid);
</pre></ul>

<p>
  The NuttX task_delete() differs from VxWorks' taskDelete() in
  the following ways:
</p>
<ul>
  <li>No support is provided for calling the tasks deletion routines (because the VxWorks <code>taskDeleteHookAdd()</code> is not supported).
  However, if <code>atexit()</code> or <code>on_exit</code> support is enabled, those will be called when the task deleted.
  <li>Deletion of self is supported, but only because <code>task_delete()</code> will re-direct processing to <code>exit()</code>.
</ul>

<H3><a name="taskrestart">2.1.5 task_restart</a></H3>
<p>
<b>Function Prototype:</b>
<ul><pre>
#include &lt;sched.h&gt;
int task_restart(pid_t pid);
</pre></ul>
<p>
  <b>Description:</b>
  This function &quot;restarts&quot; a task.
  The task is first terminated and then reinitialized with same ID, priority, original entry point, stack size, and parameters it had when it was first started.
</p>
<p>
  <b>NOTES:</b>
</p>
<ol>
  <li>
    The normal task exit clean up is not performed.
    For example, file descriptors are not closed; any files opened prior to the restart will remain opened after the task is restarted.
  </li>
  <li>
    Memory allocated by the task before it was restart is not freed.
    A task that is subject to being restart must be designed in such a way as to avoid memory leaks.
  </li>
  <li>
    Initialized data is not reset.
    All global or static data is left in the same state as when the task was terminated.
    This <i>feature</i> may be used by restart task to detect that it has been restarted, for example.
  </li>
</ol>
<p>
<b>Input Parameters:</b>
<ul>
  <li><code>pid</code>.
  The task ID of the task to delete.
  An ID of zero would signify the calling task (However, support for a task to restart itself has not been implemented).
</ul>
<p>
<b>Returned Value:</b>
<ul>
<li>
  OK, or ERROR if the task ID is invalid or the task could
  not be restarted.
  This function can fail if:
  (1) A pid of zero or the pid of the calling task is provided (functionality not implemented)
  (2) The pid is not associated with any task known to the system.
</li>
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<pre>
    STATUS taskRestart (int tid);
</pre>

<p>
  The NuttX task_restart() differs from VxWorks' taskRestart() in the following ways:
</p>
<ul>
  <li>
    Restart of the currently running task is not supported by NuttX.
  </li>
  <li>
    The VxWorks description says that the ID, priority, etc. take
    the value that they had when the task was <i>terminated</i>.
  </li>
</ul>

<H3><a name="tasksetcancelstate">2.1.6 task_setcancelstate</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;sched.h&gt;
    int task_setcancelstate(int state, int *oldstate);
</pre>
<p>
<b>Description:</b>
The <code>task_setcancelstate()</code> function atomically
sets both the calling task's cancelability state to the indicated
state and returns the previous cancelability  state at the location
referenced by oldstate.
Legal values for state are TASK_CANCEL_ENABLE and TASK_CANCEL_DISABLE.
</p>
<p>
Any pending thread cancellation may occur at the time that the
cancellation state is set to TASK_CANCEL_ENABLE.
</p>
<p>
The cancelability state and type of any newly created tasks are TASK_CANCEL_ENABLE and TASK_CANCEL_DEFERRED respectively.
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
<li><code>state</code>
New cancellation state.  One of PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE.</li>
<li><code>oldstate</code>.
Location to return the previous cancellation state.
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
Zero (<code>OK</code>) on success; <code>ERROR</code> is returned on any failure with the <code>errno</code> value set appropriately:
</p>
<p>
<ul>
<li><code>ESRCH</code>.
No thread could be found corresponding to that specified by the given thread ID.</li>
</ul>
</p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> This is a non-standard interface.  It extends the functionality of <code>pthread_setcancelstate()</code> to tasks and supports use of <code>task_delete()</code>.
</p>

<H3><a name="tasksetcanceltype">2.1.7 task_setcanceltype</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;sched.h&gt;
    int task_setcanceltype(int type, FAR int *oldtype);
</pre>
<p>
<b>Description:</b>
The <code>task_setcanceltype()</code> function atomically both sets the calling task's cancelability type to the indicated type and returns the previous cancelability type at the location referenced by <code>oldtype</code>.
Legal values for type are <code>TASK_CANCEL_DEFERRED</code> and <code>TASK_CANCEL_ASYNCHRONOUS</code>.
</p>
<p>
The cancelability state and type of any newly created tasks are <code>TASK_CANCEL_ENABLE</code> and <code>TASK_CANCEL_DEFERRED</code> respectively.
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
<li><code>type</code>
New cancellation state.  One of <code>PTHREAD_CANCEL_DEFERRED</code> or <code>PTHREAD_CANCEL_ASYNCHRONOUS</code>.</li>
<li><code>oldtype</code>.
Location to return the previous cancellation type.
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
Zero (<code>OK</code>) on success; <code>ERROR</code> is returned on any failure with the <code>errno</code> value set appropriately:
</p>
<p>
<ul>
<li><code>ESRCH</code>.
No thread could be found corresponding to that specified by the given thread ID.</li>
</ul>
</p>
<p>
<b>POSIX Compatibility:</b> This is a non-standard interface.  It extends the functionality of <code>pthread_setcanceltype()</code> to tasks and supports use of <code>task_delete()</code>.
</p>

<H3><a name="tasktestcancel">2.1.8 task_testcancel</a></H3>
<p>
<b>Function Prototype:</b>
</p>
<p>
<pre>
    #include &lt;sched.h&gt;
    void task_testcancel(void);
</pre>
</p>
<p>
<b>Description:</b>
</p>
<p>
The <code>task_testcancel()</code> function creates a <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:cancellation-points">Cancellation Point</a> in the calling task.
The <code>task_testcancel()</code> function has no effect if cancelability is disabled.
</p>
<p>
<b>Input Parameters:</b> None
</p>
<p>
<b>Returned Value:</b> None
</p>
<p>
<b>POSIX Compatibility:</b> This is a non-standard interface.  It extends the functionality of <code>pthread_testcancel()</code> to tasks and supports use of <code>task_delete()</code>.
</p>


<H3><a name="exit">2.1.9 exit</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    void exit(int code);

    #include &lt;nuttx/unistd.h&gt;
    void _exit(int code);
</pre>

<p>
<b>Description:</b> This function causes the calling task to cease
to exist -- its stack and TCB will be deallocated.  exit differs from
_exit in that it flushes streams, closes file descriptors and will
execute any function registered with <code>atexit()</code> or <code>on_exit()</code>.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>code</code>. (ignored)
</ul>

<p>
<b>Returned Value:</b>  None.

<p>
<b>Assumptions/Limitations:</b>

<p>
<b>POSIX Compatibility:</b> This is equivalent to the ANSI interface:
<pre>
    void exit(int code);
</pre>
And the UNIX interface:
<pre>
    void _exit(int code);
</pre>

<p>
  The NuttX exit() differs from ANSI exit() in the following ways:
</p>
<ul>
<li>The <code>code</code> parameter is ignored.
</ul>

<H3><a name="getpid">2.1.10 getpid</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;unistd.h&gt;
    pid_t getpid(void);
</pre>

<p>
<b>Description:</b> This function returns the task ID of the
calling task. The task ID will be invalid if called at the interrupt
level.
<p>
<b>Input Parameters:</b> None.
<p>
<b>Returned Value:</b>
<ul>
<li>The task ID of the calling task.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b>
Compatible with the POSIX interface of the same name.
</p>

<H3><a name="vfork">2.1.11 vfork</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;unistd.h&gt;
pid_t vfork(void);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>vfork()</code> function has the same effect as <code>fork()</code>, except that the behavior is undefined if the process created by <code>vfork()</code> either modifies any data other than a variable of type <code>pid_t</code> used to store the return value from <code>vfork()</code>, or returns from the function in which <code>vfork()</code> was called, or calls any other function before successfully calling <code>_exit()</code> or one of the <code>exec</code> family of functions.
<p>
<blockquote><small>
  <b>NOTE:</b>
  <code>vfork()</code> is not an independent NuttX feature, but is implemented in architecture-specific logic (using only helper functions from the NuttX core logic).
  As a result, <code>vfork()</code> may not be available on all architectures.
</small></blockquote>
<p>
  <b>Input Parameters:</b>
  None.
</p>
<p>
  <b>Returned Value:</b>
  Upon successful completion, <code>vfork()</code> returns 0 to the child process and returns
  the process ID of the child process to the parent process.
  Otherwise, -1 is returned to the parent, no child process is created, and <code>errno</code> is set to indicate the error.
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b>
  Compatible with the BSD/Linux interface of the same name.  POSIX marks this interface as Obsolete.
</p>

<H3><a name="exec">2.1.12 exec</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;nuttx/binfmt/binfmt.h&gt;
#ifndef CONFIG_BUILD_KERNEL
int exec(FAR const char *filename, FAR char * const *argv,
         FAR const struct symtab_s *exports, int nexports);
#endif
</pre></ul>
<p>
  <b>Description:</b>
  This non-standard, NuttX function is similar to <code>execv()</code> and <code>posix_spawn()</code> but differs in the following ways;
</p>
<ul>
  <li>
    Unlike <code>execv()</code> and <code>posix_spawn()</code> this function accepts symbol table information as input parameters.
    This means that the symbol table used to link the application prior to execution is provided by the caller, not by the system.
  </li>
  <li>
    Unlike <code>execv()</code>, this function always returns.
  </li>
</ul>
<p>
  This non-standard interface is included as a official NuttX API only because it is needed in certain build modes: <code>exec()</code> is probably the only want to load programs in the PROTECTED mode.  Other file execution APIs rely on a symbol table provided by the OS.  In the PROTECTED build mode, the OS cannot provide any meaningful symbolic information for execution of code in the user-space blob so that is the <code>exec()</code> function is really needed in that build case
</p>
  The interface is available in the FLAT build mode although it is not really necessary in that case.  It is currently used by some example code under the <code>apps/</code> that that generate their own symbol tables for linking test programs.  So althought it is not necessary, it can still be useful.
</p>
<p>
  The interface would be completely useless and will not be supported in the KERNEL build mode where the contrary is true:  An application process cannot provide any meaning symbolic information for use in linking a different process.
</p>
<p>
  <b>NOTE</b>:
  This function is flawed and useless without <code>CONFIG_SCHED_ONEXIT</code> and <code>CONFIG_SCHED_HAVE_PARENT</code> because without those features there is then no mechanism to unload the module once it exits.
</p>
</p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>filename</code>:
     The path to the program to be executed.
     If <code>CONFIG_LIB_ENVPATH</code> is defined in the configuration, then this may be a relative path from the current working directory.
     Otherwise, <code>path</code> must be the absolute path to the program.
   </li>
   <li>
     <code>argv</code>:
     A pointer to an array of string arguments. The end of the array is indicated with a NULL entry.
   </li>
   <li>
     <code>exports</code>:
     The address of the start of the caller-provided symbol table.
     This symbol table contains the addresses of symbols exported by the caller and made available for linking the module into the system.
   </li>
   <li>
     <code>nexports</code>:
     The number of symbols in the <code>exports</code> table.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  Zero (OK) is returned on success;
  On any failure, <code>exec()</code> will return -1 (<code>ERROR</code>) and will set the <code>errno</code> value appropriately.
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b>
  This is a non-standard inteface unique to NuttX.
  Motivation for inclusion of this non-standard interface in certain build modes is discussed above.
</p>

<H3><a name="execv">2.1.13 execv</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;unistd.h&gt;
#ifdef CONFIG_LIBC_EXECFUNCS
int execv(FAR const char *path, FAR char *const argv[]);
#endif
</pre></ul>
<p>
  <b>Description:</b>
   The standard <code>exec</code> family of functions will replace the current process image with a new process image.
   The new image will be constructed from a regular, executable file called the new process image file.
   There will be no return from a successful <code>exec</code>, because the calling process image is overlaid by the new process image.
</p>
<p>
   Simplified <code>execl()</code> and <code>execv()</code> functions are provided by NuttX for compatibility.
   NuttX is a tiny embedded RTOS that does not support processes and hence the concept of overlaying a tasks process image with a new process image does not make any sense.
   In NuttX, these functions are wrapper functions that:
</p>
<ol>
  <li>
    Call the non-standard <code>binfmt</code> function <code>exec()</code>, and then
  </li>
  <li>
     <code>exit(0)</code>.
   </li>
</ol>
<p>
   Note the inefficiency when <code>execv()</code> or <code>execl()</code> is called in the normal, two-step process:
   (1) first call <code>vfork()</code> to create a new thread, then (2) call <code>execv()</code> or <code>execl()</code> to replace the new thread with a program from the file system.
   Since the new thread will be terminated by the <code>execv()</code> or <code>execl()</code> call, it really served no purpose other than to support POSIX compatibility.
</p>
<p>
   The non-standard binfmt function <code>exec()</code> needs to have (1) a symbol table that provides the list of symbols exported by the base code, and (2) the number of symbols in that table.
   This information is currently provided to <code>exec()</code> from <code>execv()</code> or <code>execl()</code> via NuttX configuration settings:
</p>
<ul>
  <li>
     <code>CONFIG_LIBC_EXECFUNCS</code>:
     Enable <code>execv()</code> and <code>execl()</code> support
  </li>
  <li>
     <code>CONFIG_EXECFUNCS_SYMTAB_ARRAY</code>:
     Name of the ymbol table used by <code>execv()</code> or <code>execl()</code>.
  </li>
  <li>
     <code>CONFIG_EXECFUNCS_NSYMBOLS_VAR</code>:
     Name of the <code>int</code> variable holding the number of symbols in the symbol table
  </li>
</ul>
<p>
   As a result of the above, the current implementations of <code>execl()</code> and <code>execv()</code> suffer from some incompatibilities that may or may not be addressed in a future version of NuttX.
   Other than just being an inefficient use of MCU resource, the most serious of these is that
   the <code>exec</code>'ed task will not have the same task ID as the <code>vfork</code>'ed function.
   So the parent function cannot know the ID of the <code>exec</code>'ed task.<p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>path</code>:
     The path to the program to be executed.
     If <code>CONFIG_LIB_ENVPATH</code> is defined in the configuration, then this may be a relative path from the current working directory.
     Otherwise, <code>path</code> must be the absolute path to the program.
   <li>
   </li>
     <code>argv</code>:
     A pointer to an array of string arguments.
     The end of the array is indicated with a NULL entry.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  This function does not return on success.
  On failure, it will return -1 (<code>ERROR</code>) and will set the <code>errno</code> value appropriately.
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b>
  Similar with the POSIX interface of the same name.
  There are, however, several compatibility issues as detailed in the description above.
</p>

<H3><a name="execl">2.1.14 execl</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;unistd.h&gt;
#ifdef CONFIG_LIBC_EXECFUNCS
int execl(FAR const char *path, ...);
#endif
</pre></ul>
<p>
  <b>Description:</b>
  <code>execl()</code> is functionally equivalent to <a href="#execv">execv()</a>, differing only in the form of its input parameters.
  See the decription of <a href="#execv">execv()</a> for additional information.
<p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>path</code>:
     The path to the program to be executed.
     If <code>CONFIG_LIB_ENVPATH</code> is defined in the configuration, then this may be a relative path from the current working directory.
     Otherwise, <code>path</code> must be the absolute path to the program.
   <li>
   </li>
     <code>...</code>:
     A list of the string arguments to be recevied by the program.
     Zero indicates the end of the list.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  This function does not return on success.
  On failure, it will return -1 (<code>ERROR</code>) and will set the <code>errno</code> value appropriately.
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b>
  Similar with the POSIX interface of the same name.
  There are, however, several compatibility issues as detailed in the description of <a href="#execv">execv()</a>.
</p>

<h3><a name="posix_spawn">2.1.14 posix_spawn and posix_spawnp</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn(FAR pid_t *pid, FAR const char *path,
      FAR const posix_spawn_file_actions_t *file_actions,
      FAR const posix_spawnattr_t *attr,
      FAR char *const argv[], FAR char *const envp[]);
int posix_spawnp(FAR pid_t *pid, FAR const char *file,
      FAR const posix_spawn_file_actions_t *file_actions,
      FAR const posix_spawnattr_t *attr,
      FAR char *const argv[], FAR char *const envp[]);
</pre></ul>
<p>
  <b>Description:</b>
   The <code>posix_spawn()</code> and <code>posix_spawnp()</code> functions will create a new, child task, constructed from a regular executable file.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <p>
       <code>pid</code>:
       Upon successful completion, <code>posix_spawn()</code> and <code>posix_spawnp()</code> will return the task ID of the child task to the parent task, in the variable pointed to by a non-NULL <code>pid</code> argument.
       If the <code>pid</code> argument is a null pointer, the process ID of the child is not returned to the caller.
     </p>
   </li>
   <li>
     <p>
       <code>path</code> or <code>file</code>:
       The <code>path</code> argument to <code>posix_spawn()</code> is the absolute path that identifies the file to execute.
       The <code>file</code> argument to <code>posix_spawnp()</code> may also be a relative path and will be used to construct a pathname that identifies the file to execute.
       In the case of a relative path, the path prefix for the file will be obtained by a search of the directories passed as the environment variable PATH.
     </p>
     <p>
       NOTE: NuttX provides only one implementation:
       If <code>CONFIG_LIB_ENVPATH</code> is defined, then only <code>posix_spawnp()</code> behavior is supported; otherwise, only <code>posix_spawn</code> behavior is supported.
   </li>
   <li>
     <p>
       <code>file_actions</code>:
       If <code>file_actions</code> is a null pointer, then file descriptors open in the calling process will remain open in the child process (unless <code>CONFIG_FDCLONE_STDIO</code> is defined).
       If <code>file_actions</code> is not NULL, then the file descriptors open in the child process will be those open in the calling process as modified by the spawn file actions object pointed to by <code>file_actions</code>.
     </p>
   </li>
   <li>
     <p>
       <code>attr</code>:
       If the value of the <code>attr</code> parameter is <code>NULL</code>, the all default values for the POSIX spawn attributes will be used.
       Otherwise, the attributes will be set according to the spawn flags.
       The <code>posix_spawnattr_t</code> spawn attributes object type is defined in <code>spawn.h</code>.
       It will contains these attributes, not all of which are supported by NuttX:
     </p>
     <ul>
       <li>
         <code>POSIX_SPAWN_SETPGROUP</code>:
         Setting of the new task's process group is not supported.
         NuttX does not support process groups.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSCHEDPARAM</code>:
         Set new tasks priority to the <code>sched_param</code> value.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSCHEDULER</code>:
         Set the new task's scheduler policy to the <code>sched_policy</code> value.
       </li>
       <li>
         <code>POSIX_SPAWN_RESETIDS</code>
         Resetting of the effective user ID of the child process is not supported.
         NuttX does not support effective user IDs.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSIGMASK</code>:
         Set the new task's signal mask.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSIGDEF</code>:
         Resetting signal default actions is not supported.
         NuttX does not support default signal actions.
       </li>
    </ul>
  </li>
   <li>
     <p>
       <code>argv</code>:
       <code>argv[]</code> is the argument list for the new task.  <code>argv[]</code> is an array of pointers to null-terminated strings.
       The list is terminated with a null pointer.
     </p>
   </li>
   <li>
     <p>
       <code>envp</code>:
       The <code>envp[]</code> argument is not used by NuttX and may be <code>NULL</code>.
       In standard implementations, <code>envp[]</code> is an array of character pointers to null-terminated strings that provide the environment for the new process image.
       The environment array is terminated by a null pointer.
       In NuttX, the <code>envp[]</code> argument is ignored and the new task will inherit the environment of the parent task unconditionally.     </p>
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  <code>posix_spawn()</code> and <code>posix_spawnp()</code> will return zero on success.
  Otherwise, an error number will be returned as the function return value to indicate the error:
</p>
<ul>
  <li>
    <code>EINVAL</code>:
    The value specified by <code>file_actions</code> or <code>attr</code> is invalid.
  </li>
  <li>
     Any errors that might have been return if <code>vfork()</code> and <code>excec[l|v]()</code> had been called.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<ul>
  <li>
    NuttX provides only <code>posix_spawn()</code> or <code>posix_spawnp()</code> behavior depending upon the setting of <code>CONFIG_LIB_ENVPATH</code>:
    If <code>CONFIG_LIB_ENVPATH</code> is defined, then only <code>posix_spawnp()</code> behavior is supported; otherwise, only <code>posix_spawn()</code> behavior is supported.
  </li>
  <li>
    The <code>envp</code> argument is not used and the <code>environ</code> variable is not altered (NuttX does not support the <code>environ</code> variable).
  </li>
  <li>
    Process groups are not supported (See <code>POSIX_SPAWN_SETPGROUP</code> above).
  </li>
  <li>
    Effective user IDs are not supported (See <code>POSIX_SPAWN_RESETIDS</code> above).
  </li>
  <li>
    Signal default actions cannot be modified in the newly task executed  because NuttX does not support default signal actions (See <code>POSIX_SPAWN_SETSIGDEF</code>).
  </li>
</ul>
<p>
  <b>POSIX Compatibility:</b>
  The value of the <code>argv[0]</code> received by the child task is assigned by NuttX.
  For the caller of <code>posix_spawn()</code>, the provided argv[0] will correspond to <code>argv[1]</code> received by the new task.
</p>

<h3><a name="posix_spawn_file_actions_init">2.1.15 posix_spawn_file_actions_init</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn_file_actions_init(FAR posix_spawn_file_actions_t *file_actions);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawn_file_actions_init()</code> function initializes the object referenced by <code>file_actions</code> to an empty set of file actions for subsequent use in a call to <code>posix_spawn()</code> or <code>posix_spawnp()</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>file_actions</code>:
     The address of the <code>posix_spawn_file_actions_t</code> to be initialized.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>.
<p>

<h3><a name="posix_spawn_file_actions_destroy">2.1.16 posix_spawn_file_actions_destroy</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn_file_actions_destroy(FAR posix_spawn_file_actions_t *file_actions);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawn_file_actions_destroy()</code> function destroys the object referenced by <code>file_actions</code> which was previously initialized by <code>posix_spawn_file_actions_init()</code>, returning any resources obtained at the time of initialization to the system for subsequent reuse.
  A <code>posix_spawn_file_actions_t</code> may be reinitialized after having been destroyed, but must not be reused after destruction, unless it has been reinitialized.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>file_actions</code>:
     The address of the <code>posix_spawn_file_actions_t</code> to be destroyed.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
<p>

<h3><a name="posix_spawn_file_actions_addclose">2.1.17 posix_spawn_file_actions_addclose</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn_file_actions_addclose(FAR posix_spawn_file_actions_t *file_actions, int fd);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawn_file_actions_addclose()</code> function adds a <i>close</i> operation to the list of operations associated with the object referenced by <code>file_actions</code>, for subsequent use in a call to <code>posix_spawn()</code> or <code>posix_spawnp()</code>.
  The descriptor referred to by <code>fd</code> is closed as if <code>close()</code> had been called on it prior to the new child process starting execution.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>file_actions</code>:
     The address of the <code>posix_spawn_file_actions_t</code> object to which the <i>close</i> operation will be appended.
   </li>
   <li>
     <code>fd</code>:
     The file descriptor to be closed.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawn_file_actions_adddup2">2.1.18 posix_spawn_file_actions_adddup2</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn_file_actions_adddup2(FAR posix_spawn_file_actions_t *file_actions, int fd1, int fd2);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawn_file_actions_adddup2()</code> function adds a <i>dup2</i> operation to the list of operations associated with the object referenced by <code>file_actions</code>, for subsequent use in a call to <code>posix_spawn()</code> or <code>posix_spawnp()</code>.
  The descriptor referred to by <code>fd2</code> is created as if <code>dup2()</code> had been called on <code>fd1</code> prior to the new child process starting execution.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>file_actions</code>:
     The address of the <code>posix_spawn_file_actions_t</code> object to which the <i>dup2</i> operation will be appended.
   </li>
   <li>
     <code>fd1</code>:
     The file descriptor to be be duplicated.
     The first file descriptor to be argument to <code>dup2()</code>.
   </li>
   <li>
     <code>fd2</code>:
     The file descriptor to be be created.
     The second file descriptor to be argument to <code>dup2()</code>.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawn_file_actions_addopen">2.1.19 posix_spawn_file_actions_addopen</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn_file_actions_addopen(FAR posix_spawn_file_actions_t *file_actions,
      int fd, FAR const char *path, int oflags, mode_t mode);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawn_file_actions_addopen()</code> function adds an <i>open</i> operation to the list of operations associated with the object referenced by <code>file_actions</code>, for subsequent use in a call to <code>posix_spawn()</code> or <code>posix_spawnp()</code>.
  The descriptor referred to by <code>fd</code> is opened using the <code>path</code>, <code>oflag</code>, and <code>mode</code> arguments as if <code>open()</code> had been called on it prior to the new child process starting execution.
  The string path is copied by the <code>posix_spawn_file_actions_addopen()</code> function during this process, so storage need not be persistent in the caller.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>file_actions</code>:
     The address of the <code>posix_spawn_file_actions_t</code> object to which the <i>open</i> operation will be appended.
   </li>
   <li>
     <code>fd</code>:
     The file descriptor to be opened.
   </li>
   <li>
     <code>path</code>:
     The path to be opened.
   </li>
   <li>
     <code>oflags</code>:
     Open flags.
   </li>
   <li>
     <code>mode</code>:
     File creation mode/
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_init">2.1.20 posix_spawnattr_init</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_init(FAR posix_spawnattr_t *attr);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_init()</code> function initializes the object referenced by <code>attr</code>, to an empty set of spawn attributes for subsequent use in a call to  <code>posix_spawn()</code> or  <code>posix_spawnp()</code>.
<p>
<p>
  Then the spawn attributes are no longer needed, they should be destroyed by calling <code>posix_spawnattr_destroyed()</code>.
  In NuttX, however, <code>posix_spawnattr_destroyed()</code> is just stub:
</p>
<ul><pre>
#define posix_spawnattr_destroy(attr) (0)
</pre></ul>
<p>
  For portability, the convention of calling <code>posix_spawnattr_destroyed()</code> when the attributes are not longer needed should still be followed.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address of the spawn attributes to be initialized.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_getflags">2.1.21 posix_spawnattr_getflags</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_getflags(FAR const posix_spawnattr_t *attr, FAR short *flags);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_getflags()</code> function will obtain the value of the <i>spawn-flags</i> attribute from the attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be queried.
   </li>
   <li>
     <code>flags</code>:
      The location to return the spawn flags
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_getschedparam">2.1.22 posix_spawnattr_getschedparam</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_getschedparam(FAR const posix_spawnattr_t *attr, FAR struct sched_param *param);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_getschedparam()</code> function will obtain the value of the <i>spawn-schedparam</i> attribute from the attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be queried.
   </li>
   <li>
     <code>param</code>:
      The location to return the <i>spawn-schedparam</i> value.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_getschedpolicy">2.1.23 posix_spawnattr_getschedpolicy</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_getschedpolicy(FAR const posix_spawnattr_t *attr, FAR int *policy);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_getschedpolicy()</code> function will obtain the value of the <i>spawn-schedpolicy</i> attribute from the attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be queried.
   </li>
   <li>
     <code>policy</code>:
      The location to return the <i>spawn-schedpolicy</i> value.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_getsigmask">2.1.24 posix_spawnattr_getsigmask</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;

int posix_spawnattr_getsigmask(FAR const posix_spawnattr_t *attr, FAR sigset_t *sigmask);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_getsigdefault()</code> function will obtain the value of the <i>spawn-sigmask</i> attribute from the attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be queried.
   </li>
   <li>
     <code>sigmask</code>:
      The location to return the <i>spawn-sigmask</i> value.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_setflags">2.1.25 posix_spawnattr_setflags</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_setflags(FAR posix_spawnattr_t *attr, short flags);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_setflags()</code> function will set the <i>spawn-flags</i> attribute in an initialized attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be used.
   </li>
   <li>
     <code>flags</code>:
      The new value of the <i>spawn-flags</i> attribute.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_setschedparam">2.1.26 posix_spawnattr_setschedparam</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_setschedparam(FAR posix_spawnattr_t *attr, FAR const struct sched_param *param);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_setschedparam()</code> function will set the <i>spawn-schedparam</i> attribute in an initialized attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be used.
   </li>
   <li>
     <code>param</code>:
      The new value of the <i>spawn-schedparam</i> attribute.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_setschedpolicy">2.1.27 posix_spawnattr_setschedpolicy</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawnattr_setschedpolicy(FAR posix_spawnattr_t *attr, int policy);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_setschedpolicy()</code> function will set the <i>spawn-schedpolicy</i> attribute in an initialized attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be used.
   </li>
   <li>
     <code>policy</code>:
      The new value of the <i>spawn-schedpolicy</i> attribute.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawnattr_setsigmask">2.1.28 posix_spawnattr_setsigmask</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;

int posix_spawnattr_setsigmask(FAR posix_spawnattr_t *attr, FAR const sigset_t *sigmask);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawnattr_setsigmask()</code> function will set the <i>spawn-sigmask</i> attribute in an initialized attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be used.
   </li>
   <li>
     <code>sigmask</code>:
      The new value of the <i>spawn-sigmask</i> attribute.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="task_spawn">2.1.29 task_spawn</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int task_spawn(FAR pid_t *pid, FAR const char *name, main_t entry,
      FAR const posix_spawn_file_actions_t *file_actions,
      FAR const posix_spawnattr_t *attr,
      FAR char *const argv[], FAR char *const envp[]);
</pre></ul>
<p>
  <b>Description:</b>
   The <code>task_spawn()</code> function will create a new, child task, where the entry point to the task is an address in memory.
</p>
</p>
<ul>
   <li>
     <p>
       <code>pid</code>:
       Upon successful completion, <code>task_spawn()</code> will return the task ID of the child task to the parent task, in the variable pointed to by a non-NULL <code>pid</code> argument.
       If the <code>pid</code> argument is a null pointer, the process ID of the child is not returned to the caller.
     </p>
   </li>
   <li>
     <p>
       <code>name</code>:
       The name to assign to the child task.
     </p>
   </li>
   <li>
     <p>
       <code>entry</code>:
       The child task's entry point (an address in memory).
     </p>
   </li>
   <li>
     <p>
       <code>file_actions</code>:
       If <code>file_actions</code> is a null pointer, then file descriptors open in the calling process will remain open in the child process (unless <code>CONFIG_FDCLONE_STDIO</code> is defined).
       If <code>file_actions</code> is not NULL, then the file descriptors open in the child process will be those open in the calling process as modified by the spawn file actions object pointed to by <code>file_actions</code>.
     </p>
   </li>
   <li>
     <p>
       <code>attr</code>:
       If the value of the <code>attr</code> parameter is <code>NULL</code>, the all default values for the POSIX spawn attributes will be used.
       Otherwise, the attributes will be set according to the spawn flags.
       The <code>posix_spawnattr_t</code> spawn attributes object type is defined in <code>spawn.h</code>.
       It will contains these attributes, not all of which are supported by NuttX:
     </p>
     <ul>
       <li>
         <code>POSIX_SPAWN_SETPGROUP</code>:
         Setting of the new task's process group is not supported.
         NuttX does not support process groups.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSCHEDPARAM</code>:
         Set new tasks priority to the <code>sched_param</code> value.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSCHEDULER</code>:
         Set the new task's scheduler policy to the <code>sched_policy</code> value.
       </li>
       <li>
         <code>POSIX_SPAWN_RESETIDS</code>
         Resetting of the effective user ID of the child process is not supported.
         NuttX does not support effective user IDs.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSIGMASK</code>:
         Set the new task's signal mask.
       </li>
       <li>
         <code>POSIX_SPAWN_SETSIGDEF</code>:
         Resetting signal default actions is not supported.
         NuttX does not support default signal actions.
       </li>
    </ul>
    <p>
       And the non-standard:
    </p>
    <ul>
      <li>
        <code>TASK_SPAWN_SETSTACKSIZE</code>:
        Set the stack size for the new task.
      </li>
    </ul>
  </li>
   <li>
     <p>
       <code>argv</code>:
       <code>argv[]</code> is the argument list for the new task.  <code>argv[]</code> is an array of pointers to null-terminated strings.
       The list is terminated with a null pointer.
     </p>
   </li>
   <li>
     <p>
       <code>envp</code>:
       The <code>envp[]</code> argument is not used by NuttX and may be <code>NULL</code>.
     </p>
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  <code>task_spawn()</code> will return zero on success.
  Otherwise, an error number will be returned as the function return value to indicate the error:
</p>
<p>
  <b>POSIX Compatibility:</b>
  This is a non-standard interface inspired by <code>posix_spawn()</code>.
</p>

<h3><a name="task_spawnattr_getstacksize">2.1.30 task_spawnattr_getstacksize</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int task_spawnattr_getstacksize(FAR const posix_spawnattr_t *attr, FAR size_t *stacksize);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>task_spawnattr_getstacksize()</code> function will obtain the value of the <i>spawn-stacksize</i> attribute from the attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be queried.
   </li>
   <li>
     <code>policy</code>:
      The location to return the <i>spawn-stacksize</i> value.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="task_spawnattr_setstacksize">2.1.31 task_spawnattr_setstacksize</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int task_spawnattr_setstacksize(FAR posix_spawnattr_t *attr, size_t stacksize);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>task_spawnattr_setstacksize()</code> function will set the <i>spawn-stacksize</i> attribute in an initialized attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>attr</code>:
     The address spawn attributes to be used.
   </li>
   <li>
     <code>policy</code>:
      The new value of the <i>spawn-stacksize</i> attribute.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>
</p>

<h3><a name="posix_spawn_file_actions_init">2.1.32 posix_spawn_file_actions_init</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
  <ul><pre>
#include &lt;spawn.h&gt;
int posix_spawn_file_actions_init(FAR posix_spawn_file_actions_t *file_actions);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>posix_spawn_file_actions_init()</code> function initializes the object referenced by <code>file_actions</code> to an empty set of file actions for subsequent use in a call to <code>posix_spawn()</code> or <code>posix_spawnp()</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
   <li>
     <code>file_actions</code>:
     The address of the <code>posix_spawn_file_actions_t</code> to be initialized.
   </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, this function returns 0; on failure it will return an error number from <code>&lt;errno.h&gt;</code>.
<p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Task_Schedule"><h2>2.2 Task Scheduling Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  By default, NuttX performs strict priority scheduling: Tasks of higher priority have exclusive access to the CPU until they become blocked.
  At that time, the CPU is available to tasks of lower priority.
  Tasks of equal priority are scheduled FIFO.
</p>
<p>
  Optionally, a Nuttx task or thread can be configured with round-robin or <i>sporadic</i> scheduler.
  The round-robin is similar to priority scheduling <i>except</i> that tasks with equal priority and share CPU time via <i>time-slicing</i>.
  The time-slice interval is a constant determined by the configuration
  setting <code>CONFIG_RR_INTERVAL</code> to a positive, non-zero value.
  Sporadic scheduling scheduling is more complex, varying the priority of a thread over a <i>replenishment</i> period.
  Support for sporadic scheduling is enabled by the configuration option <code>CONFIG_SCHED_SPORADIC</code>.
</p>
<p>
  The OS interfaces described in the following paragraphs provide a POSIX- compliant interface to the NuttX scheduler:
</p>
<ul>
  <li><a href="#schedsetparam">2.2.1 sched_setparam</a></li>
  <li><a href="#schedgetparam">2.2.2 sched_getparam</a></li>
  <li><a href="#schedsetscheduler">2.2.3 sched_setscheduler</a></li>
  <li><a href="#setgetscheduler">2.2.4 sched_getscheduler</a></li>
  <li><a href="#sched_yield">2.2.5 sched_yield</a></li>
  <li><a href="#schedgetprioritymax">2.2.6 sched_get_priority_max</a></li>
  <li><a href="#schedgetprioritymin">2.2.7 sched_get_priority_min</a></li>
  <li><a href="#schedgetrrinterval">2.2.8 sched_get_rr_interval</a></li>
</ul>

<H3><a name="schedsetparam">2.2.1 sched_setparam</a></H3>
<p>
  <b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_setparam(pid_t pid, const struct sched_param *param);
</pre>
<p>
  <b>Description:</b>
  This function sets the priority of the task specified by pid input parameter.
</p>
<p>
  NOTE: Setting a task's priority to the same value has the similar
  effect to <code>sched_yield()</code>: The task will be moved to after all
  other tasks with the same priority.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>pid</code>.
    The task ID of the task.
    If <code>pid</code> is zero, the priority of the calling task is set.
  </li>
  <li>
    <code>param</code>.
    A structure whose member <code>sched_priority</code> is the integer priority.
    The range of valid priority numbers is from <code>SCHED_PRIORITY_MIN</code> through <code>SCHED_PRIORITY_MAX</code>.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, sched_setparam() returns 0 (<code>OK</code>).
  On error, -1 (<code>ERROR</code>) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately.
</p>
<ul>

  <li>
    <code>EINVAL</code>.
     The parameter <code>param</code> is invalid or does not make sense for the current scheduling policy.
  </li>
  <li>
    <code>EPERM</code>.
    The calling task does not have appropriate privileges.
  </li>
  <li>
    <code>ESRCH</code>.
    The task whose ID is <code>pid</code> could not be found.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name.
  Differences from the full POSIX implementation include:
</p>
<ul>
  <li>The range of priority values for the POSIX call is 0 to 255.</li>
</ul>

<H3><a name="schedgetparam">2.2.2 sched_getparam</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_getparam (pid_t pid, struct sched_param *param);
</pre>

<p>
<b>Description:</b> This function gets the scheduling priority
of the task specified by pid.
<p>
<b>Input Parameters:</b>
<ul>
<li>
  <code>pid</code>. The task ID of the task.
  If pid is zero, the priority of the calling task is returned.
</li>
<li>
  <code>param</code>.
  A structure whose member <code>sched_priority</code> is the integer priority.
   The task's priority is copied to the <code>sched_priority</code> element of this structure.
</li>
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) if successful, otherwise -1 (<code>ERROR</code>).
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="schedsetscheduler">2.2.3 sched_setscheduler</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sched.h&gt;
int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param);
</pre></ul>
<p>
  <b>Description:</b>
  <code>sched_setscheduler()</code>  sets both the scheduling policyand the priority for the task identified by <code>pid</code>.
  If <code>pid</code> equals zero, the scheduler of the calling thread will be set.
  The parameter <code>param</code> holds the priority of the thread under the new policy.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>pid</code>.
    The task ID of the task.
    If <code>pid</code> is zero, the priority of the calling task is set.
  </li>
  <li>
    <code>policy</code>.
    Scheduling policy requested (either <code>SCHED_FIFO</code> or <code>SCHED_RR</code>).
  </li>
  <li>
    <code>param</code>.
    A structure whose member <code>sched_priority</code> is the integer priority.
    The range of valid priority numbers is from <code>SCHED_PRIORITY_MIN</code> through <code>SCHED_PRIORITY_MAX</code>.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, <code>sched_setscheduler()</code> returns <code>OK</code> (zero).  On
  error, <code>ERROR</code> (-1) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
  <li><code>EINVAL</code>: The scheduling <code>policy</code> is not one of the recognized policies.</li>
  <li><code>ESRCH</code>: The task whose ID is <code>pid</code> could not be found.</li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="setgetscheduler">2.2.4 sched_getscheduler</a></H3>
<p>
<b>Function Prototype:</b>
<ul><pre>
#include &lt;sched.h&gt;
int sched_getscheduler (pid_t pid);
</pre></ul>
<p>
  <b>Description:</b>
  <code>sched_getscheduler()</code> returns the scheduling policy
  currently applied to the task identified by <code>pid</code>. If
  <code>pid</code> equals zero, the policy of the calling process will
  be retrieved.
<p>
<b>Input Parameters:</b>
<ul>
  <li><code>pid</code>.
    The task ID of the task to query.
    If <code>pid</code> is zero, the calling task is queried.
  </LI>
</ul>
<p>
<b>Returned Value:</b>
<ul>
  <li>
    On success, <code>sched_getscheduler()</code> returns the policy for
    the task (either <code>SCHED_FIFO</code> or <code>SCHED_RR</code>).
    On error,  <code>ERROR</code> (-1) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
    <ul>
      <li><code>ESRCH</code>: The task whose ID is pid could not be found.</li>
    </ul>
  </li>
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX interface of the same name.

<H3><a name="sched_yield">2.2.5 sched_yield</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_yield(void);
</pre>

<p>
<b>Description:</b> This function forces the calling task to give
up the CPU (only to other tasks at the same priority).
<p>
<b>Input Parameters:</b> None.
<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) or -1 (<code>ERROR</code>)
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="schedgetprioritymax">2.2.6 sched_get_priority_max</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_get_priority_max (int policy)
</pre>

<p>
<b>Description:</b> This function returns the value of the highest
possible task priority for a specified scheduling policy.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>policy</code>. Scheduling policy requested.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>The maximum priority value or -1 (<code>ERROR</code>).
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="schedgetprioritymin">2.2.7 sched_get_priority_min</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_get_priority_min (int policy);
</pre>

<p>
<b>Description:</b> This function returns the value of the lowest
possible task priority for a specified scheduling policy.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>policy</code>. Scheduling policy requested.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>The minimum priority value or -1 (<code>ERROR</code>)
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="schedgetrrinterval">2.2.8 sched_get_rr_interval</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_get_rr_interval (pid_t pid, struct timespec *interval);
</pre>

<p>
  <b>Description:</b>
  <code>sched_rr_get_interval()</code>  writes the timeslice interval
  for task identified by <code>pid</code> into the timespec structure
  pointed to by <code>interval</code>.  If pid is zero, the timeslice
  for the calling process is written into 'interval.  The
  identified process should be running under the SCHED_RR
  scheduling policy.'
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
<li><code>pid</code>. The task ID of the task. If pid is zero, the
priority of the calling task is returned.
<li><code>interval</code>. A structure used to return the time slice.
</ul>

<p>
  <b>Returned Value:</b>
  On success, sched_rr_get_interval() returns OK (0).  On
  error, ERROR (-1) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set to:
</p>
<ul>
  <li><code>EFAULT</code>: Cannot copy to interval</LI>
  <li><code>EINVAL</code>: Invalid pid.</LI>
  <li><code>ENOSYS</code>: The system call is not yet implemented.</LI>
  <li><code>ESRCH</code>: The process whose ID is pid could not be found.</LI>
</ul>

<p>
  <b>Assumptions/Limitations:</b>
<p>
  <b>POSIX  Compatibility:</b> Comparable to the POSIX
  interface of the same name.
</P>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Task_Switch"><h2>2.3 Task Control Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  <b>Task Control Interfaces</b>.
</p>
<ul>
  <li>
    <p>
      <b>Scheduler locking interfaces</b>.
      This <i>non-standard</i> interfaces are used to enable and disable pre-emption and to test is pre-emption is currently enabled.
    </p>
    <ul>
      <li><a href="#schedlock">2.3.1 sched_lock</a></li>
      <li><a href="#schedunlock">2.3.2 sched_unlock</a></li>
      <li><a href="#schedlockcount">2.3.3 sched_lockcount</a></li>
    </ul>
  </li>
  <li>
    <p>
      <b>Task synchronization interfaces</b>.
      <code>wait()</code>, <code>waitpid()</code> or <code>waitid()</code> may be used to wait for termination of child tasks.
    </p>
    <ul>
      <li><a href="#waitpid">2.3.4 waitpid</a></li>
      <li><a href="#waitid">2.3.5 waitid</a></li>
      <li><a href="#wait">2.3.6 wait</a></li>
    </ul>
  <li>
    <p>
      <b>Task Exit Hooks</b>.
      <code>atexit()</code> and <code>on_exit()</code> may be use to register callback functions that are executed when a <i>task group</i> terminates.
      A task group is the functional analog of a process:
      It is a group that consists of the main task thread and of all of the pthreads created by the main task thread or any of the other pthreads within the task group.
      Members of a task group share certain resources such as environment variables, file descriptors, <code>FILE</code> streams, sockets, pthread keys and open message queues.
    </p>
    <blockquote><small>
      <b>NOTE:</b>
      Behavior of features related to <i>task group</i>s depend of NuttX configuration settings.
      See the discussion of &quot;Parent and Child Tasks,&quot; below.
      See also the <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:nxtasking">NuttX Threading Wiki</a> page and the <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:tasksnthreads">Tasks vs. Threads FAQ</a> for additional information on tasks and threads in NuttX.
    </small></blockquote>
    <p>
      A <i>task group</i> terminates when the last thread within the group exits.
    </p>
    <ul>
      <li><a href="#atexit">2.3.7 atexit</a></li>
      <li><a href="#onexit">2.3.8 on_exit</a></li>
    </ul>
  </li>
</ul>

<p>
  <b>Parent and Child Tasks</b>.
  The task synchronization interfaces historically depend upon parent and child relationships between tasks.
  But default, NuttX does not use any parent/child knowledge.
  However, there are three important configuration options that can change that.
</p>
<ul>
  <li>
    <p>
      <b><code>CONFIG_SCHED_HAVE_PARENT</code></b>.
      If this setting is defined, then it instructs NuttX to remember the task ID of the parent task when each new child task is created.
      This support enables some additional features (such as <code>SIGCHLD</code>) and modifies the behavior of other interfaces.
      For example, it makes <code>waitpid()</code> more standards complete by restricting the waited-for tasks to the children of the caller.
    </p>
  </li>
  <li>
    <p>
      <b><code>CONFIG_SCHED_CHILD_STATUS</code></b>
      If this option is selected, then the exit status of the child task will be retained after the child task exits.
      This option should be selected if you require knowledge of a child process' exit status.
      Without this setting, <code>wait()</code>, <code>waitpid()</code> or <code>waitid()</code> may fail.
      For example, if you do:
    </p>
    <ol>
      <li>
        Start child task
      </li>
      <li>
        Wait for exit status (using <code>wait()</code>, <code>waitpid()</code> or <code>waitid()</code>).
      </li>
    </ol>
    <p>
      This may fail because the child task may run to completion before the wait begins.
      There is a non-standard work-around in this case:
      The above sequence will work if you disable pre-emption using <code>sched_lock()</code> prior to starting the child task, then re-enable pre-emption with <code>sched_unlock()</code> after the wait completes.
      This works because the child task is not permitted to run until the wait is in place.
    </p>
    <p>
      The standard solution would be to enable <code>CONFIG_SCHED_CHILD_STATUS</code>.
      In this case the exit status of the child task is retained after the child exits and the wait will successful obtain the child task's exit status whether it is called before the child task exits or not.
    </p>
  </li>
  <li>
    <p>
      <b><code>CONFIG_PREALLOC_CHILDSTATUS</code></b>.
      To prevent runaway child status allocations and to improve allocation performance, child task exit status structures are pre-allocated when the system boots.
      This setting determines the number of child status structures that will be pre-allocated.
      If this setting is not defined or if it is defined to be zero then a value of 2*<code>MAX_TASKS</code> is used.
    </p>
    <p>
      Note that there cannot be more that <code>CONFIG_MAX_TASKS</code> tasks in total.
      However, the number of child status structures may need to be significantly larger because this number includes the maximum number of tasks that are running PLUS the number of tasks that have exit'ed without having their exit status reaped (via <code>wait()</code>, <code>waitpid()</code> or <code>waitid()</code>).
    </p>
    <p>
      Obviously, if tasks spawn children indefinitely and never have the exit status reaped, then you may have a memory leak!
      (See <b>Warning</b> below)
    </p>
  </li>
</ul>
<p>
  <b>Warning</b>:
  If you enable the <code>CONFIG_SCHED_CHILD_STATUS</code> feature, then your application must either (1) take responsibility for reaping the child status with <code>wait()</code>, <code>waitpid()</code> or <code>waitid()</code>, or (2) suppress retention of child status.
  If you do not reap the child status, then you have a memory leak and your system will eventually fail.
</p>
  Retention of child status can be suppressed on the parent using logic like:
</p>
<ul><pre>
struct sigaction sa;

sa.sa_handler = SIG_IGN;
sa.sa_flags = SA_NOCLDWAIT;
int ret = sigaction(SIGCHLD, &sa, NULL);
</pre></ul>

<H3><a name="schedlock">2.3.1 sched_lock</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_lock(void);
</pre>

<p>
<b>Description:</b> This function disables context switching by
Disabling addition of new tasks to the ready-to-run task list.
The task that calls this function will be the only task that is
allowed to run until it either calls sched_unlock (the appropriate
number of times) or until it blocks itself.
<p>
<b>Input Parameters:</b> None.
<p>
<b>Returned Value:</b>
<ul>
<li>OK or ERROR.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX  Compatibility:</b> This is a NON-POSIX interface.
VxWorks provides the comparable interface:
<pre>
    STATUS taskLock(void);
</pre>

<H3><a name="schedunlock">2.3.2 sched_unlock</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int sched_unlock(void);
</pre>

<p>
<b>Description:</b> This function decrements the preemption lock
count. Typically this is paired with sched_lock() and concludes
a critical section of code. Preemption will not be unlocked until
sched_unlock() has been called as many times as sched_lock().
When the lockCount is decremented to zero, any tasks that were
eligible to preempt the current task will execute.
<p>
<b>Input Parameters:</b> None.
<p>
<b>Returned Value:</b>
<ul>
<li>OK or ERROR.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX  Compatibility:</b> This is a NON-POSIX interface.
VxWorks provides the comparable interface:
<pre>
    STATUS taskUnlock(void);
</pre>

<H3><a name="schedlockcount">2.3.3 sched_lockcount</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sched.h&gt;
    int32_t sched_lockcount(void)
</pre>

<p>
<b>Description:</b> This function returns the current value of
the lockCount. If zero, preemption is enabled; if non-zero, this
value indicates the number of times that sched_lock() has been called
on this thread of execution.
<p>
<b>Input Parameters:</b> None.
<p>
<b>Returned Value:</b>
<ul>
<li>The current value of the lockCount.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> None.
</p>

<H3><a name="waitpid">2.3.4 waitpid</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sys/wait.h&gt;
    ipid_t waitpid(pid_t pid, int *stat_loc, int options);
</pre>

<p>
  <b>Description:</b>
</p>
<blockquote><small>
  The following discussion is a general description of the <code>waitpid()</code> interface.
  However, as of this writing, the implementation of <code>waitpid()</code> is incomplete (but usable).
  If <code>CONFIG_SCHED_HAVE_PARENT</code> is defined, <code>waitpid()</code> will be a little more compliant to specifications.
  Without <code>CONFIG_SCHED_HAVE_PARENT</code>, <code>waitpid()</code> simply supports waiting for any task to complete execution.
  With <code>CONFIG_SCHED_HAVE_PARENT</code>, <code>waitpid()</code> will use <code>SIGCHLD</code> and can, therefore, wait for any child of the parent to complete.
  The implementation is incomplete in either case, however: NuttX does not support any concept of process groups.
  Nor does NuttX retain the status of exited tasks so if <code>waitpid()</code> is called after a task has exited, then no status will be available.
  The options argument is currently ignored.
</small></blockquote>
<p>
  The <code>waitpid()</code> functions will obtain status information pertaining to one of the caller's child processes.
  The <code>waitpid()</code> function will suspend execution of the calling thread until status information for one of the terminated child processes of the calling process is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process.
  If more than one thread is suspended in <code>waitpid()</code> awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination.
  If status information is available prior to the call to <code>waitpid()</code>, return will be immediate.
</p>
<p>
  <b>NOTE</b>:
  Because <code>waitpid()</code> is not fully POSIX compliant, it must be specifically enabled by setting <code>CONFIG_SCHED_WAITPID</code> in the NuttX configuration file.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
<li><code>pid</code>. The task ID of the thread to waid for</li>
<li><code>stat_loc</code>. The location to return the exit status</li>
<li><code>options</code>. ignored</li>
</ul>
<p>
  The <code>pid</code> argument specifies a set of child processes for which status is requested.
  The <code>waitpid()</code> function will only return the status of a child process from this set:
</p>
<ul>
  <li>
    If <code>pid</code> is equal to <code>(pid_t)-1</code>), status is requested for any child process.
    In this respect, <code>waitpid()</code> is then equivalent to  <code>wait()</code>.
  </li>
  <li>
    If <code>pid</code> is greater than 0, it specifies the process ID of a single child process for which status is requested.
  </li>
  <li>
    If <code>pid</code> is 0, status is requested for any child process whose process group ID is equal to that of the calling process.
  </li>
  <li>
    If <code>pid</code> is less than <code>(pid_t)-1</code>), status is requested for any child process whose process group ID is equal to the absolute value of pid.
  </li>
</ul>
<p>
  The <code>options</code> argument is constructed from the bitwise-inclusive OR of zero or more of the following flags,
  defined in the <code>&lt;sys/wait.h&gt;</code> header:
</p>
<ul>
  <li>
    <code>WCONTINUED</code>.
    The <code>waitpid()</code> function will report the status of any continued child process specified by pid whose status has not been reported since it continued from a job control stop.
  </li>
  <li>
    <code>WNOHANG</code>.
    The <code>waitpid()</code> function will not suspend execution of the calling thread if status is not immediately available for one of the child processes specified by <code>pid</code>.
  </li>
  <li>
    <code>WUNTRACED</code>.
    The status of any child processes specified by <code>pid</code> that are stopped, and whose status has not yet been reported since they stopped, will also be reported to the requesting process.
  </li>
</ul>
<p>
  If the calling process has <code>SA_NOCLDWAIT</code> set or has <code>SIGCHLD</code> set to <code>SIG_IGN</code>, and the process has no unwaited-for children that were transformed into zombie processes, the calling thread will block until all of the children of the process containing the calling thread terminate, and <code>waitpid()</code> will fail and set <code>errno</code> to <code>ECHILD</code>.
</p>
<p>
  If <code>waitpid()</code> returns because the status of a child process is available, these functions will return a value equal to the process ID of the child process.
  In this case, if the value of the argument stat_loc is not a null pointer, information will be stored in the location pointed to by <code>stat_loc</code>.
  The value stored at the location pointed to by <code>stat_loc</code> will be 0 if and only if the status returned is from a terminated child process that terminated by one of the following means:
</p>
<ol>
  <li>
    The process returned 0 from <code>main()</code>.
  </li>
  <li>
    The process called <code>_exit()</code> or <code>exit()</code> with a status argument of 0.
  </li>
  <li>
    The process was terminated because the last thread in the process terminated.
  </li>
</ol>
<p>
  Regardless of its value, this information may be interpreted using the following macros, which are defined in <code>&lt;sys/wait.h&gt;</code> and evaluate to integral expressions; the <code>stat_val</code> argument is the integer value pointed to by <code>stat_loc</code>.
</p>
<ul>
  <li>
    <code>WIFEXITED(stat_val)</code>.
    Evaluates to a non-zero value if status was returned for a child process that terminated normally.
  </li>
  <li>
    <code>WEXITSTATUS(stat_val)</code>.
    If the value of <code>WIFEXITED(stat_val)</code> is non-zero, this macro evaluates to the low-order 8 bits of the status argument that the child process passed to <code>_exit()</code> or <code>exit()</code>, or the value the child process returned from <code>main()</code>.
  </li>
  <li>
    <code>WIFSIGNALED(stat_val)</code>.
    Evaluates to a non-zero value if status was returned for a child process that terminated due to the receipt of a signal that was not caught (see &gt;signal.h&lt;).
  </li>
  <li>
    <code>WTERMSIG(stat_val)</code>.
    If the value of <code>WIFSIGNALED(stat_val)</code> is non-zero, this macro evaluates to the number of the signal that caused the termination of the child process.
  </li>
  <li>
    <code>WIFSTOPPED(stat_val)</code>.
    Evaluates to a non-zero value if status was returned for a child process that is currently stopped.
  </li>
  <li>
    <code>WSTOPSIG(stat_val)</code>.
    If the value of <code>WIFSTOPPED(stat_val)</code> is non-zero, this macro evaluates to the number of the signal that caused the child process to stop.
  </li>
  <li>
    <code>WIFCONTINUED(stat_val)</code>.
    Evaluates to a non-zero value if status was returned for a child process that has continued from a job control stop.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<p>
  If <code>waitpid()</code> returns because the status of a child process is available, it will return a value equal to the process ID of the child process for which status is reported.
</p>
<p>
  If <code>waitpid()</code> returns due to the delivery of a signal to the calling process, -1 will be returned and <code>errno</code> set to <code>EINTR</code>.
</p>
<p>
  If <code>waitpid()</code> was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, 0 is returned.
</p>
<p>
  Otherwise, <code>(pid_t)-1</code< will be returned, and <code>errno</code> set to indicate the error:
</p>
<ul>
  <li>
    <code>ECHILD</code>.
    The process specified by <code>pid</code> does not exist or is not a child of the calling process, or the process group specified by <code>pid</code> does not exist or does not have any member process that is a child of the calling process.
  </li>
  <li>
    <code>EINTR</code>.
    The function was interrupted by a signal.
    The value of the location pointed to by <code>stat_loc</code> is undefined.
  </li>
  <li>
    <code>EINVAL</code>.
    The <code>options</code> argument is not valid.
  </li>
</ul>

<p>
  <b>Assumptions/Limitations:</b>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name, but the implementation is incomplete (as detailed above).
</P>

<h3><a name="waitid">2.3.5 waitid</a></li></h3>
<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sys/wait.h&gt;
    #ifdef CONFIG_SCHED_HAVE_PARENT
    int waitid(idtype_t idtype, id_t id, FAR siginfo_t *info, int options);
    #endif
</pre>
<p>
  <b>Description:</b>
</p>
<blockquote><small>
  The following discussion is a general description of the <code>waitid()</code> interface.
  However, as of this writing, the implementation of <code>waitid()</code> is incomplete (but usable).
  If <code>CONFIG_SCHED_HAVE_PARENT</code> is defined, <code>waitid()</code> will be a little more compliant to specifications.
  <code>waitpid()</code> simply supports waiting a specific child task (<code>P_PID</code> or for any child task <code>P_ALL</code> to complete execution.
  <code>SIGCHLD</code> is used.
  The implementation is incomplete in either case, however: NuttX does not support any concept of process groups.
  Nor does NuttX retain the status of exited tasks so if <code>waitpid()</code> is called after a task has exited, then no status will be available.
  The options argument is currently ignored.
</small></blockquote>
<p>
  The <code>waitid()</code> function suspends the calling thread until one child of the process containing the calling thread changes state.
  It records the current state of a child in the structure pointed to by <code>info</code>.
  If a child process changed state prior to the call to <code>waitid()</code>, <code>waitid()</code> returns immediately.
  If more than one thread is suspended in <code>wait()</code> or <code>waitpid()</code> waiting termination of the same process, exactly one thread will return the process status at the time of the target process termination
</p>
<p>
  The <code>idtype</code> and <code>id</code> arguments are used to specify which children <code>waitid()</code> will wait for.
</p>
<p>
<ul>
  <li>
   If <code>idtype</code> is P_PID, <code>waitid()</code> will wait for the child with a process ID equal to (pid_t)<code>id</code>.
  </li>
  <li>
   If <code>idtype</code> is P_PGID, <code>waitid()</code> will wait for any child with a process group ID equal to (pid_t)<code>id</code>.
  </li>
  <li>
   If <code>idtype</code> is P_ALL, <code>waitid()</code> will wait for any children and <code>id</code> is ignored.
</ul>
<p>
  The <code>options</code> argument is used to specify which state changes <code>waitid()</code> will will wait for.
  It is formed by OR-ing together one or more of the following flags:
</p>
<ul>
  <li>
    <code>WEXITED</code>:
    Wait for processes that have exited.
  </li>
  <li>
    <code>WSTOPPED</code>:
    Status will be returned for any child that has stopped upon receipt of a signal.
  </li>
  <li>
    <code>WCONTINUES</code>:
    Status will be returned for any child that was stopped and has been continued.
  </li>
  <li>
    <code>WNOHANG</code>:
    Return immediately if there are no children to wait for.
  </li>
  <li>
    <code>WNOWAIT</code>:
    Keep the process whose status is returned in <code>info</code> in a waitable state.
    This will not affect the state of the process;
    the process may be waited for again after this call completes.
  </li>
</ul>
  The <code>info</code> argument must point to a <code>siginfo_t</code> structure.
  If <code>waitid()</code> returns because a child process was found that satisfied the conditions indicated by the arguments <code>idtype</code> and options, then the structure pointed to by <code>info</code> will be filled in by the system with the status of the process.
  The <code>si_signo</code> member will always be equal to <code>SIGCHLD</code>.
</p>
<p>
  <b>Input Parameters:</b>
  See the description above.
</p>
<p>
  <b>Returned Value:</b>
  If <code>waitid()</code> returns due to the change of state of one of its children, 0 is returned.
  Otherwise, -1 is returned and <code>errno</code> is set to indicate the error.
</p>
<p>
  The <code>waitid()</code> function will fail if:
</p>
<ul>
  <li>
    <code>ECHILD</code>:
  </li>
    The calling process has no existing unwaited-for child processes.
  <li>
    <code>EINTR</code>:
  </li>
    The <code>waitid()</code> function was interrupted by a signal.
  <li>
    <code>EINVAL</code>:
     An invalid value was specified for <code>options</code>, or <code>idtype</code> and <code>id</code> specify an invalid set of processes.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name, but the implementation is incomplete (as detailed in the description above).
</p>

<h3><a name="wait">2.3.6 wait</a></li></h3>
<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;sys/wait.h&gt;
    #ifdef CONFIG_SCHED_HAVE_PARENT
    pid_t wait(FAR int *stat_loc);
    #endif
</pre>
<p>
  <b>Description:</b>
</p>
<blockquote><small>
  The following discussion is a general description of the <code>wait()</code> interface.
  However, as of this writing, the implementation of <code>wait()</code> is incomplete (but usable).
  <code>wait()</code> is based on <a href="#waitpid"><code>waitpaid()</code></a>.
  See the description of <a href="#waitpid"><code>waitpaid()</code></a> for further information.
</small></blockquote>
<p>
  The <code>wait()</code> function will suspend execution of the calling thread until status information for one of its terminated child processes is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process.
  If more than one thread is suspended in <code>wait()</code> awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination.
  If status information is available prior to the call to<code>wait()</code>, return will be immediate.
</p>
<p>
  The <code>waitpid()</code> function will behave identically to <code>wait()</code>, if its <code>pid</code> argument is (pid_t)-1 and the options argument is 0.
  Otherwise, its behavior will be modified by the values of the <code>pid</code> and <code>options</code> arguments.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>stat_loc</code>. The location to return the exit status</li>
</ul>
<p>
  <b>Returned Value:</b>
  See the values returned by <a href="#waitpid"><code>waitpaid()</code></a>.
</p>
<p>
  <b>Assumptions/Limitations:</b>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name, but the implementation is incomplete (as detailed in the description <a href="#waitpid"><code>waitpaid()</code></a>).
</p>

<h3><a name="atexit">2.3.7 atexit</a></h3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;stdlib.h&gt;
    int atexit(void (*func)(void));
</pre>
<p>
  <b>Description:</b>
   Registers a function to be called at program exit.
   The <code>atexit()</code> function registers the given function to be called at normal process termination, whether via <code>exit()</code> or via return from the program's <code>main()</code>.
</p>
<p>
   <b>NOTE</b>: <code>CONFIG_SCHED_ATEXIT</code> must be defined to enable this function.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
<li><code>func</code>. A pointer to the function to be called when the task exits.</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, <code>atexit()</code> returns OK (0).
  On error, ERROR (-1) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set to indicate the cause of the failure.
</p>

<p>
  <b>Assumptions/Limitations:</b>
<p>
  <b>POSIX  Compatibility:</b> Comparable to the ISO C interface of the same name.
   Limitiations in the current implementation:
</p>
<ol>
  <li>Only a single <code>atexit</code> function can be registered unless <code>CONFIG_SCHED_ATEXIT_MAX</code> defines a larger number.</li>
  <li><code>atexit()</code> functions are not inherited when a new task is created.</li>
</ol>

<H3><a name="onexit">2.3.8 on_exit</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;stdlib.h&gt;
    int on_exit(CODE void (*func)(int, FAR void *), FAR void *arg)
</pre>
<p>
  <b>Description:</b>
   Registers a function to be called at program exit.
   The <code>on_exit()</code> function registers the given function to be called at normal process termination, whether via <code>exit()</code> or via return from the program's <code>main()</code>.
   The function is passed the status argument given to the last call to <code>exit()</code> and the <code>arg</code> argument from <code>on_exit()</code>.
</p>
<p>
   <b>NOTE</b>: <code>CONFIG_SCHED_ONEXIT</code> must be defined to enable this function
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
<li><code>func</code>. A pointer to the function to be called when the task exits.</li>
<li><code>arg</code>. An argument that will be provided to the <code>on_exit()</code> function when the task exits.</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, <code>on_exit()</code> returns OK (0).
  On error, ERROR (-1) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set to indicate the cause of the failure.
</p>

<p>
  <b>Assumptions/Limitations:</b>
<p>
  <b>POSIX  Compatibility:</b>
  This function comes from SunOS 4, but is also present in libc4, libc5 and glibc.
  It no longer occurs in Solaris (SunOS 5).
  Avoid this function, and use the standard <code>atexit()</code> instead.
</p>
<ol>
  <li>Only a single <code>on_exit</code> function can be registered unless <code>CONFIG_SCHED_ONEXIT_MAX</code> defines a larger number.</li>
  <li><code>on_exit()</code> functions are not inherited when a new task is created.</li>
</ol>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Message_Queue"><h2>2.4 Named Message Queue Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  NuttX supports POSIX named message queues for inter-task communication.
  Any task may send or receive messages on named message queues.
  Interrupt handlers may send messages via named message queues.
</p>
<ul>
  <li><a href="#mqopen">2.4.1 mq_open</a></li>
  <li><a href="#mqclose">2.4.2 mq_close</a></li>
  <li><a href="#mqunlink">2.4.3 mq_unlink</a></li>
  <li><a href="#mqsend">2.4.4 mq_send</a></li>
  <li><a href="#mqtimedsend">2.4.5 mq_timedsend</a></li>
  <li><a href="#mqreceive">2.4.6 mq_receive</a></li>
  <li><a href="#mqtimedreceive">2.4.7 mq_timedreceive</a></li>
  <li><a href="#mqnotify">2.4.8 mq_notify</a></li>
  <li><a href="#mqsetattr">2.4.9 mq_setattr</a></li>
  <li><a href="#mqgetattr">2.4.10 mq_getattr</a></li>
</ul>

<H3><a name="mqopen">2.4.1 mq_open</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;mqueue.h&gt;
    mqd_t mq_open(const char *mqName, int oflags, ...);
</pre>

<p>
<b>Description:</b> This function establish a connection between
a named message queue and the calling task. After a successful
call of mq_open(), the task can reference the message queue using
the address returned by the call. The message queue remains usable
until it is closed by a successful call to mq_close().
<p>
<b>Input Parameters:</b>
<ul>
<li><code>mqName</code>. Name of the queue to open
<li><code>oflags</code>. Open flags. These may be any combination of:
<ul>
<li><code>O_RDONLY</code>. Open for read access.
<li><code>O_WRONLY</code>. Open for write access.
<li><code>O_RDWR</code>. Open for both read &amp; write access.
<li><code>O_CREAT</code>. Create message queue if it does not already
exist.
<li><code>O_EXCL</code>. Name must not exist when opened.
<li><code>O_NONBLOCK</code>. Don't wait for data.
</ul>

<li><i><code>...</code> Optional parameters</i>.
When the O_CREAT flag is specified, POSIX requires that a third
and fourth parameter be supplied:
<ul>
<li><code>mode</code>.  The mode parameter is of type mode_t.  In the POSIX
specification, this mode value provides file permission bits for the
message queue.  This parameter is required but not used in the present
implementation.
<li><code>attr</code>.  A pointer to an mq_attr that is provided to initialize.
the message queue.  If attr is NULL, then the messages queue is created
with implementation-defined default message queue attributes.  If attr is
non-NULL, then the message queue mq_maxmsg attribute is set to the
corresponding value when the queue is created.  The mq_maxmsg attribute
determines the maximum number of messages that can be queued before
addition attempts to send messages on the message queue fail or cause the
sender to block; the mq_msgsize attribute determines the maximum size of a
message that can be sent or received.  Other elements of attr are ignored
(i.e, set to default message queue attributes).
</ul>
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>A message queue descriptor or -1 (<code>ERROR</code>)
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX interface
of the same name.
Differences from the full POSIX implementation include:
<ul>
<li>The mq_msgsize attributes determines the maximum size of a message that
may be sent or received.  In the present implementation, this maximum
message size is limited at 22 bytes.
</ul>

<H3><a name="mqclose">2.4.2 mq_close</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_close(mqd_t mqdes);
</pre>

<p>
<b>Description:</b> This function is used to indicate that the
calling task is finished with the specified message queued mqdes.
The mq_close() deallocates any system resources allocated by the
system for use by this task for its message queue.
<p>
If the calling task has attached a notification request to the message
queue via this <code>mqdes</code> (see <code>mq_notify()</code>), this attachment will be
removed and the message queue is available for another task to attach
for notification.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>mqdes</code>. Message queue descriptor.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) if the message queue is closed successfully, otherwise,
-1 (<code>ERROR</code>).
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<ul>
  <li>
    The behavior of a task that is blocked on either a <code>mq_send()</code> or <code>mq_receive()</code> is undefined when <code>mq_close()</code> is called.
  </li>
  <li>
    The result of using this message queue descriptor after successful return from <code>mq_close()</code> is undefined.
  </li>
</ul>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX interface
of the same name.

<H3><a name="mqunlink">2.4.3 mq_unlink</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_unlink(const char *mqName);
</pre>

<p>
<b>Description:</b> This function removes the message queue named
by &quot;mqName.&quot; If one or more tasks have the message queue
open when <code>mq_unlink()</code> is called, removal of the message queue
is postponed until all references to the message queue have been
closed.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>mqName</code>. Name of the message queue
</ul>

<p>
<b>Returned Value:</b> None.
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="mqsend">2.4.4 mq_send</a></H3>
<p>
<b>Function Prototype:</b>
</p>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_send(mqd_t mqdes, const void *msg, size_t msglen, int prio);
</pre>
<p>
<b>Description:</b>
  This function adds the specified message, <code>msg</code>, to the message queue, <code>mqdes</code>.
  The <code>msglen</code> parameter specifies the length of the message in bytes pointed to by <code>msg</code>.
  This length must not exceed the maximum message length from the <code>mq_getattr()</code>.
</p>
<p>
  If the message queue is not full, <code>mq_send()</code> will place the <code>msg</code>
  in the message queue at the position indicated by the <code>prio</code> argument.
  Messages with higher priority will be inserted before lower priority messages
  The value of <code>prio</code> must not exceed <code>MQ_PRIO_MAX</code>.
</p>
<p>
  If the specified message queue is full and <code>O_NONBLOCK</code> is not
  set in the message queue, then <code>mq_send()</code> will block until space
  becomes available to the queue the message.
</p>
<p>
  If the message queue is full and <code>NON_BLOCK</code> is set, the message
  is not queued and <code>ERROR</code> is returned.
</p>
<p>
  <b>NOTE</b>:
  <code>mq_send()</code> may be called from an interrupt handler.
  However, it behaves differently when called from the interrupt level:
</p>
<ul>
  <li>
    It does not check the size of the queue.
    It will always post the message, even if there is already too many messages in queue.
    This is because the interrupt handler does not have the option of waiting for the message queue to become non-full.
  </li>
  <li>
    It doesn't allocate new memory (because you cannot allocate memory from an interrup handler).
    Instead, there are are pool of pre-allocated message structures that may be used just for sending messages from interrupt handlers.
    The number of such pre-allocated messages is a configuration parameter.
  </li>
</ul>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>mqdes</code>. Message queue descriptor.</li>
  <li><code>msg</code>. Message to send.</li>
  <li><code>msglen</code>. The length of the message in bytes.</li>
  <li><code>prio</code>. The priority of the message.</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, <code>mq_send()</code> returns 0 (<code>OK</code>);
  on error, -1 (<code>ERROR</code>) is returned, with <a href="#ErrnoAccess"><code>errno</code></a> set
  to indicate the error:
</p>
<ul>
  <li>
    <code>EAGAIN</code>.
    The queue was empty, and the <code>O_NONBLOCK</code> flag was set for the message queue description referred to by <code>mqdes</code>.
  </li>
  <li>
    <code>EINVAL</code>.
    Either <code>msg</code> or <code>mqdes</code> is <code>NULL</code> or the value of <code>prio</code> is invalid.
  </li>
  <li>
    <code>EPERM</code>.
    Message queue opened not opened for writing.
  </li>
  <li>
    <code>EMSGSIZE</code>.
    <code>msglen</code> was greater than the <code>maxmsgsize</code> attribute of the message queue.
  </li>
  <li>
    <code>EINTR</code>.
    The call was interrupted by a signal handler.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<h3><a name="mqtimedsend">mq_timedsend</a></h3>
<b>Function Prototype:</b>
</p>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_timedsend(mqd_t mqdes, const char *msg, size_t msglen, int prio,
                     const struct timespec *abstime);
</pre>
<p>
<b>Description:</b>
  This function adds the specified message, <code>msg</code>,
  to the message queue, <code>mqdes</code>.
  The <code>msglen</code> parameter specifies the length of the message in bytes pointed to by <code>msg</code>.
  This length must not exceed the maximum message length from the <code>mq_getattr()</code>.
</p>
<p>
  If the message queue is not full, <code>mq_timedsend()</code> will place the <code>msg</code>
  in the message queue at the position indicated by the <code>prio</code> argument.
  Messages with higher priority will be inserted before lower priority messages
  The value of <code>prio</code> must not exceed <code>MQ_PRIO_MAX</code>.
</p>
<p>
  If the specified message queue is full and <code>O_NONBLOCK</code> is not
  set in the message queue, then <code>mq_send()</code> will block until space
  becomes available to the queue the message or until a timeout occurs.
</p>
<p>
  <code>mq_timedsend()</code> behaves just like <code>mq_send()</code>, except
  that if the queue is full and the <code>O_NONBLOCK</code> flag is not enabled
  for the message queue description, then <code>abstime</code> points to a
  structure which specifies a ceiling on the time for which the call will block.
  This ceiling is an absolute timeout in seconds and nanoseconds since the
  Epoch (midnight on the morning of 1 January 1970).
</p>
<p>
  If the message queue is full, and the timeout has already expired by the time
  of the call, <code>mq_timedsend()</code> returns immediately.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>mqdes</code>. Message queue descriptor.</li>
  <li><code>msg</code>. Message to send.</li>
  <li><code>msglen</code>. The length of the message in bytes.</li>
  <li><code>prio</code>. The priority of the message.</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, <code>mq_send()</code> returns 0 (<code>OK</code>);
  on error, -1 (<code>ERROR</code>) is returned, with <a href="#ErrnoAccess"><code>errno</code></a> set
  to indicate the error:
</p>
<ul>
  <li>
    <code>EAGAIN</code>.
    The queue was empty, and the <code>O_NONBLOCK</code> flag was set for the message queue description referred to by <code>mqdes</code>.
  </li>
  <li>
    <code>EINVAL</code>.
    Either <code>msg</code> or <code>mqdes</code> is <code>NULL</code> or the value of <code>prio</code> is invalid.
  </li>
  <li>
    <code>EPERM</code>.
    Message queue opened not opened for writing.
  </li>
  <li>
    <code>EMSGSIZE</code>.
    <code>msglen</code> was greater than the <code>maxmsgsize</code> attribute of the message queue.
  </li>
  <li>
    <code>EINTR</code>.
    The call was interrupted by a signal handler.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<h3><a name="mqreceive">2.4.5 mq_receive</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;mqueue.h&gt;
    ssize_t mq_receive(mqd_t mqdes, void *msg, size_t msglen, int *prio);
</pre>
<p>
  <b>Description:</b>
  This function receives the oldest of the highest priority messages from the message
  queue specified by <code>mqdes</code>.
  If the size of the buffer in bytes, <code>msgLen</code>, is less than the
  <code>mq_msgsize</code> attribute of the message queue, <code>mq_receive()</code> will
  return an error.
  Otherwise, the selected message is removed from the queue and copied to <code>msg</code>.
</p>
<p>
  If the message queue is empty and <code>O_NONBLOCK</code> was not set, <code>mq_receive()</code>
  will block until a message is added to the message queue.
  If more than one task is waiting to receive a message, only the task with the highest
  priority that has waited the longest will be unblocked.
</p>
<p>
  If the queue is empty and <code>O_NONBLOCK</code> is set, <code>ERROR</code> will be returned.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>mqdes</code>. Message Queue Descriptor.</li>
  <li><code>msg</code>. Buffer to receive the message.</li>
  <li><code>msglen</code>. Size of the buffer in bytes.</li>
  <li><code>prio</code>. If not NULL, the location to store message priority.
</ul>
<p>
  <b>Returned Value:</b>.
  One success, the length of the selected message in bytes is returned.
  On failure, -1 (<code>ERROR</code>) is returned and the <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
  <li>
    <code>EAGAIN</code>
    The queue was empty and the <code>O_NONBLOCK</code> flag was set for the message queue description referred to by <code>mqdes</code>.
  </li>
  <li>
    <code>EPERM</code>
    Message queue opened not opened for reading.
  </li>
  <li>
    <code>EMSGSIZE</code>
    <code>msglen</code> was less than the <code>maxmsgsize</code> attribute of the message queue.
  </li>
  <li>
    <code>EINTR</code>
    The call was interrupted by a signal handler.
  </li>
  <li>
    <code>EINVAL</code>
    Invalid <code>msg</code> or <code>mqdes</code>
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<h3><a name="mqtimedreceive">2.4.6 mq_timedreceive</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;mqueue.h&gt;
    ssize_t mq_timedreceive(mqd_t mqdes, void *msg, size_t msglen,
                            int *prio, const struct timespec *abstime);
</pre>
<p>
  <b>Description:</b>
  This function receives the oldest of the highest priority messages from the message
  queue specified by <code>mqdes</code>.
  If the size of the buffer in bytes, <code>msgLen</code>, is less than the
  <code>mq_msgsize</code> attribute of the message queue, <code>mq_timedreceive()</code> will
  return an error.
  Otherwise, the selected message is removed from the queue and copied to <code>msg</code>.
</p>
<p>
  If the message queue is empty and <code>O_NONBLOCK</code> was not set, <code>mq_timedreceive()</code>
  will block until a message is added to the message queue (or until a timeout occurs).
  If more than one task is waiting to receive a message, only the task with the highest
  priority that has waited the longest will be unblocked.
</p>
<p>
  <code>mq_timedreceive()</code> behaves just like <code>mq_receive()</code>, except
  that if the queue is empty and the <code>O_NONBLOCK</code> flag is not enabled
  for the message queue description, then <code>abstime</code> points to a structure
  which specifies a ceiling on the time for which the call will block.
  This ceiling is an absolute timeout in seconds and nanoseconds since the Epoch
  (midnight on the morning of 1 January 1970).
</p>
<p>
  If no message is available, and the timeout has already expired by the time of
  the call, <code>mq_timedreceive()</code> returns immediately.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>mqdes</code>. Message Queue Descriptor.</li>
  <li><code>msg</code>. Buffer to receive the message.</li>
  <li><code>msglen</code>. Size of the buffer in bytes.</li>
  <li><code>prio</code>. If not NULL, the location to store message priority.
  <li><code>abstime</code>. The absolute time to wait until a timeout is declared.
</ul>
<p>
  <b>Returned Value:</b>.
  One success, the length of the selected message in bytes is returned.
  On failure, -1 (<code>ERROR</code>) is returned and the <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
  <li>
    <code>EAGAIN</code>:
    The queue was empty and the <code>O_NONBLOCK</code> flag was set for the message queue description referred to by <code>mqdes</code>.
  </li>
  <li>
    <code>EPERM</code>:
    Message queue opened not opened for reading.
  </li>
  <li>
    <code>EMSGSIZE</code>:
    <code>msglen</code> was less than the <code>maxmsgsize</code> attribute of the message queue.
  </li>
  <li>
    <code>EINTR</code>:
    The call was interrupted by a signal handler.
  </li>
  <li>
    <code>EINVAL</code>:
    Invalid <code>msg</code> or <code>mqdes</code> or <code>abstime</code>
  </li>
  <li>
    <code>ETIMEDOUT</code>:
    The call timed out before a message could be transferred.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<h3><a name="mqnotify">2.4.7 mq_notify</a></h3>

<p>
  <b>Function Prototype:</b>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_notify(mqd_t mqdes, const struct sigevent *notification);
</pre>
</p>
<p>
  <b>Description:</b> If the <code>notification</code> input parameter
  is not <code>NULL</code>, this function connects the task with the message queue such
  that the specified signal will be sent to the task whenever the message
  changes from empty to non-empty. One notification can be attached
  to a message queue.
</p>
<p>
  If <code>notification</code>; is <code>NULL</code>, the attached notification
  is detached (if it was held by the calling task) and the queue
  is available to attach another notification.
</p>
<p>
  When the notification is sent to the registered task, its registration
  will be removed.  The message queue will then be available for
  registration.
<p>
  <b>Input Parameters:</b>
  <ul>
    <li>
      <code>mqdes</code>. Message queue descriptor
    </li>
    <li><code>notification</code>. Real-time signal structure containing:
    <ul>
      <li><code>sigev_notify</code>. Should be SIGEV_SIGNAL (but actually ignored)
      <li><code>sigev_signo</code>. The signo to use for the notification
      <li><code>sigev_value</code>. Value associated with the signal
    </ul>
  </ul>
</p>
<p>
  <b>Returned Value:</b>
  On success <code>mq_notify()</code> returns 0; on error, -1 is returned, with
  <code>errno</code> set to indicate the error:
  <ul>
    <li>
      <code>EBADF</code>. The descriptor specified in <code>mqdes</code> is invalid.
    </li>
    <li>
      <code>EBUSY</code>. Another process has already registered to receive notification
      for this message queue.
    </li>
    <li>
      <code>EINVAL</code>. <code>sevp->sigev_notify</code> is not one of the permitted values; or
      <code>sevp->sigev_notify</code> is <code>SIGEV_SIGNAL</code> and <code>sevp->sigev_signo</code> is not a
      valid signal number.
    </li>
    <li>
      <code>ENOMEM</code>. Insufficient memory.
    </li>
  </ul>
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b> Comparable to the POSIX interface of the same name.
  Differences from the full POSIX implementation include:
  <ul>
    <li>
      The notification signal will be sent to the registered task even if
      another task is waiting for the message queue to become non-empty.  This is
      inconsistent with the POSIX specification which states, &quot;If a process
      has registered for notification of message arrival at a message queue and
      some process is blocked in <code>mq_receive</code> waiting to receive a message
      when a message arrives at the queue, the arriving message will satisfy the
      appropriate <code>mq_receive()</code> ... The resulting behavior is as if the
      message queue remains empty, and no notification will be sent.&quot;
    </li>
  </ul>
</p>

<H3><a name="mqsetattr">2.4.8 mq_setattr</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_setattr(mqd_t mqdes, const struct mq_attr *mqStat,
                   struct mq_attr *oldMqStat);
</pre>

<p>
<b>Description:</b> This function sets the attributes associated
with the specified message queue &quot;mqdes.&quot; Only the &quot;O_NONBLOCK&quot;
bit of the &quot;mq_flags&quot; can be changed.
<p>
If &quot;oldMqStat&quot; is non-null, mq_setattr() will store
the previous message queue attributes at that location (just as
would have been returned by mq_getattr()).
<p>
<b>Input Parameters:</b>
<ul>
<li><code>mqdes</code>. Message queue descriptor
<li><code>mqStat</code>. New attributes
<li><code>oldMqState</code>. Old attributes
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) if attributes are set successfully, otherwise -1
(<code>ERROR</code>).
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="mqgetattr">2.4.9 mq_getattr</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;mqueue.h&gt;
    int mq_getattr(mqd_t mqdes, struct mq_attr *mqStat);
</pre>

<p>
<b>Description:</b> This functions gets status information and
attributes associated with the specified message queue.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>mqdes</code>. Message queue descriptor
<li><code>mqStat</code>. Buffer in which to return attributes. The returned
attributes include:
<ul>
<li><code>mq_maxmsg</code>. Max number of messages in queue.
<li><code>mq_msgsize</code>. Max message size.
<li><code>mq_flags</code>. Queue flags.
<li><code>mq_curmsgs</code>. Number of messages currently in queue.
</ul>

</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) if attributes provided, -1 (<code>ERROR</code>) otherwise.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Semaphores"><h2>2.5 Counting Semaphore Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  <b>Semaphores</b>.  Semaphores are the basis for
  synchronization and mutual exclusion in NuttX. NuttX supports
  POSIX semaphores.
</p>
<p>
  Semaphores are the preferred mechanism for gaining exclusive access to a
  resource.  sched_lock() and sched_unlock() can also be used for this purpose.
  However, sched_lock() and sched_unlock() have other undesirable side-affects
  in the operation of the system:  sched_lock() also prevents higher-priority
  tasks from running that do not depend upon the semaphore-managed resource
  and, as a result, can adversely affect system response times.
</p>
<p>
  <a name="priorityinversion"><b>Priority Inversion</b></a>.
  Proper use of semaphores avoids the issues of <code>sched_lock()</code>.
  However, consider the following example:
  <OL>
    <li>Some low-priority task, <i>Task C</i>, acquires a semaphore in order to
      get exclusive access to a protected resource.</li>
    <li><i>Task C</i> is suspended to allow some high-priority task,</li>
      <i>Task A</i>, to execute.</li>
    <li><i>Task A</i> attempts to acquire the semaphore held by <i>Task C</i> and
      gets blocked until <i>Task C</i> relinquishes the semaphore.</li>
    <li><i>Task C</i> is allowed to execute again, but gets suspended by some
      medium-priority <i>Task B</i>.</li>
  </OL>
<p>
  At this point, the high-priority <i>Task A</i> cannot execute until
  <i>Task B</i> (and possibly other medium-priority tasks) completes and until
  <i>Task C</i> relinquishes the semaphore.  In effect, the high-priority task,
  <i>Task A</i> behaves as though it were lower in priority than the
  low-priority task, <i>Task C</i>!  This phenomenon is called <i>priority
  inversion</i>.
</p>
<p>
  Some operating systems avoid priority inversion by <i>automatically</i>
  increasing the priority of the low-priority <i>Task C</i> (the operable
  buzz-word for this behavior is <i>priority inheritance</i>).  NuttX
  supports this behavior, but only if <code>CONFIG_PRIORITY_INHERITANCE</code>
  is defined in your OS configuration file.  If <code>CONFIG_PRIORITY_INHERITANCE</code>
  is not defined, then it is left to the designer to provide implementations
  that will not suffer from priority inversion.
  The designer may, as examples:
</p>
<ul>
  <li>Implement all tasks that need the semaphore-managed resources at the
    same priority level,</li>
  <li>Boost the priority of the low-priority task before the semaphore is
    acquired, or</li>
  <li>Use sched_lock() in the low-priority task.</li>
</ul>
<p>
  <a name="priorityinheritance"><b>Priority Inheritance</b></a>.
  As mentioned, NuttX does support <i>priority inheritance</i> provided that
  <code>CONFIG_PRIORITY_INHERITANCE</code> is defined in your OS configuration file.
  However, the implementation and configuration of the priority inheritance feature
  is sufficiently complex that more needs to be said.
  How can a feature that can be described by a single, simple sentence require such
  a complex implementation:
</p>
<ul>
  <li>
    <b><code>CONFIG_SEM_PREALLOCHOLDERS</code>.</b>
    First of all, in NuttX priority inheritance is implement on POSIX counting
    semaphores.  The reason for this is that these semaphores are the most
    primitive waiting mechanism in NuttX; Most other waiting facilities are
    based on semaphores.  So if priority inheritance is implemented for POSIX
    counting semaphores, then most NuttX waiting mechanisms will have this
    capability.
    <p>
      Complexity arises because counting semaphores can have numerous
      holders of semaphore counts.  Therefore, in order to implement
      priority inheritance across all holders, then internal data
      structures must be allocated to manage the various holders associated
      with a semaphore.
      The setting <code>CONFIG_SEM_PREALLOCHOLDERS</code> defines the maximum
      number of different threads (minus one per semaphore instance) that can
      take counts on a semaphore with priority inheritance support.
      This setting defines the size of a single pool of pre-allocated structures.
      It may be set to zero if priority inheritance is disabled OR if you
      are only using semaphores as mutexes (only one holder) OR if no more
      than two threads participate using a counting semaphore.
    </p>
    <p>
      The cost associated with setting <code>CONFIG_SEM_PREALLOCHOLDERS</code>
      is slightly increased code size and around 6-12 bytes times the value
      of <code>CONFIG_SEM_PREALLOCHOLDERS</code>.
    </p>
  </li>
  <li>
    <b><code>CONFIG_SEM_NNESTPRIO</code>:</b>
    In addition, there may be multiple threads of various priorities that
    need to wait for a count from the semaphore.
    These, the lower priority thread holding the semaphore may have to
    be boosted numerous time and, to make things more complex, will have
    to keep track of all of the boost priorities values in in order to
    correctly restore the priorities after a count has been handed out
    to the higher priority thread.
    The <code>CONFIG_SEM_NNESTPRIO</code> defines the size of an array,
    one array per active thread.
    This setting is the maximum number of higher priority threads (minus
    1) than can be waiting for another thread to release a count on a semaphore.
    This value may be set to zero if no more than one thread is expected to
    wait for a semaphore.
    <p>
      The cost associated with setting <code>CONFIG_SEM_NNESTPRIO</code>
      is slightly increased code size and (<code>CONFIG_SEM_PREALLOCHOLDERS</code> + 1)
      times the maximum number of active threads.
    </p>
  </li>
  <li>
    <b>Increased Susceptibility to Bad Thread Behavior</b>.
    These various structures tie the semaphore implementation more tightly to
    the behavior of the implementation.  For examples, if a thread executes while
    holding counts on a semaphore, or if a thread exits without call <code>sem_destroy()</code>
    then.  Or what if the thread with the boosted priority re-prioritizes itself?
    The NuttX implement of priority inheritance attempts to handle all of these
    types of corner cases, but it is very likely that some are missed.
    The worst case result is that memory could by stranded within the priority
    inheritance logic.
  </li>
</ul>
<p>
  <a name="lockingvssignaling"><b>Locking versus Signaling Semaphores</b></a>.
  Semaphores (and mutexes) may be used for many different purposes.
  One typical use is for mutual exclusion and locking of resources:
  In this usage, the thread that needs exclusive access to a resources takes the semaphore to get access to the resource.
  The same thread subsequently releases the seamphore count when it no longer needs exclusive access.
  Priority inheritance is intended just for this usage case.
</p>
<p>
  In a different usage case, a semaphore may to be used to signal an event:
  One thread A waits on a semaphore for an event to occur.
  When the event occurs, another thread B will post the semaphore waking the waiting thread A.
  This is a completely different usage model; notice that in the mutual exclusion case, the same thread takes and posts the semaphore.  In the signaling case, one thread takes the seamphore and a different thread posts the samphore.  Priority inheritance should <i>never</i> be used in this signaling case.
  Subtle, strange behaviors may result.
</p>
<p>
  When priority inheritance is enabled with <code>CONFIG_PRIORITY_INHERITANCE</code>, the default <i>protocol</i> for the semaphore will be to use priority inheritance.
  For signaling semaphores, priority inheritance must be explicitly disabled by calling <a href="#semsetprotocol"><code>sem_setprotocol</code></a> with <code>SEM_PRIO_NONE</code>.
  For the case of pthread mutexes, <a href="#pthreadmutexattrsetprotocol"><code>pthread_mutexattr_setprotocol</code></a> with <code>PTHREAD_PRIO_NONE</code>.
</p>
<p>
  This is discussed in much more detail on this <a href="http://www.nuttx.org/doku.php?id=wiki:howtos:signalling-semaphores">Wiki page</a>.
</p>
<p>
  <b>POSIX semaphore interfaces:</b>
</p>
<ul>
  <li><a href="#seminit">2.5.1 sem_init</a></li>
  <li><a href="#semdestroy">2.5.2 sem_destroy</a></li>
  <li><a href="#semopen">2.5.3 sem_open</a></li>
  <li><a href="#semclose">2.5.4 sem_close</a></li>
  <li><a href="#semunlink">2.5.5 sem_unlink</a></li>
  <li><a href="#semwait">2.5.6 sem_wait</a></li>
  <li><a href="#semtimedwait">2.5.7 sem_timedwait</a></li>
  <li><a href="#semtrywait">2.5.8 sem_trywait</a></li>
  <li><a href="#sempost">2.5.9 sem_post</a></li>
  <li><a href="#semgetvalue">2.5.10 sem_getvalue</a></li>
  <li><a href="#semgetprotocol">2.5.11 sem_getprotocol</a></li>
  <li><a href="#semsetprotocol">2.5.12 sem_setprotocol</a></li>
</ul>

<H3><a name="seminit">2.5.1 sem_init</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_init(sem_t *sem, int pshared, unsigned int value);
</pre>

<p>
<b>Description:</b> This function initializes the UN-NAMED semaphore
sem. Following a successful call to sem_init(), the semaphore
may be used in subsequent calls to sem_wait(), sem_post(), and
sem_trywait(). The semaphore remains usable until it is destroyed.
<p>
Only <code>sem</code> itself may be used for performing synchronization.  The
result of referring to copies of <code>sem</code> in calls to <code>sem_wait()</code>,
<code>sem_trywait()</code>, <code>sem_post()</code>, and <code>sem_destroy()</code>, is
not defined.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. Semaphore to be initialized
<li><code>pshared</code>. Process sharing (not used)
<li><code>value</code>. Semaphore initialization value
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the full POSIX implementation include:
<ul>
<li>pshared is not used.
</ul>

<H3><a name="semdestroy">2.5.2 sem_destroy</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_destroy(sem_t *sem);
</pre>

<p>
<b>Description:</b> This function is used to destroy the un-named semaphore
indicated by <code>sem</code>.  Only a semaphore that was created using
<code>sem_init()</code> may be destroyed using <code>sem_destroy()</code>.  The effect
of calling <code>sem_destroy()</code> with a named semaphore is undefined.  The
effect of subsequent use of the semaphore <code>sem</code> is undefined until
<code>sem</code> is re-initialized by another call to <code>sem_init()</code>.
<p>
The effect of destroying a semaphore upon which other tasks are currently
blocked is undefined.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. Semaphore to be destroyed.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="semopen">2.5.3 sem_open</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    sem_t *sem_open(const char *name, int oflag, ...);
</pre>

<p>
<b>Description:</b> This function establishes a connection between
named semaphores and a task. Following a call to sem_open() with
the semaphore name, the task may reference the semaphore associated
with name using the address returned by this call. The semaphore
may be used in subsequent calls to sem_wait(), sem_trywait(),
and sem_post(). The semaphore remains usable until the semaphore
is closed by a successful call to sem_close().
<p>
If a task makes multiple calls to sem_open() with the same name,
then the same semaphore address is returned (provided there have
been no calls to sem_unlink()).
<p>
<b>Input Parameters:</b>
<ul>
<li><code>name</code>. Semaphore name
<li><code>oflag</code>. Semaphore creation options. This may one of
the following bit settings:
<ul>
<li><code>oflag</code> = 0: Connect to the semaphore only if it already
exists.
<li><code>oflag</code> = O_CREAT: Connect to the semaphore if it exists,
otherwise create the semaphore.
<li><code>oflag</code> = O_CREAT with O_EXCL (O_CREAT|O_EXCL): Create
a new semaphore unless one of this name already exists.
</ul>
<li>... Optional parameters.
NOTE:  When the O_CREAT flag is specified, POSIX requires that a third
and fourth parameter be supplied:
<ul>
<li><code>mode</code>.  The mode parameter is of type mode_t.
This parameter is required but not used in the present
implementation.
<li><code>value</code>.  The value parameter is type unsigned int.  The semaphore
is created with an initial value of <code>value</code>.  Valid initial values for
semaphores must be less than or equal to <code>SEM_VALUE_MAX</code> (defined in
<CODE>include/limits.h</CODE>).
</ul>
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>A pointer to sem_t or <code>SEM_FAILED</code> if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the full POSIX implementation include:
<ul>
<li>Treatment of links/connections is highly simplified. It is
just a counting semaphore.
</ul>

<H3><a name="semclose">2.5.4 sem_close</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_close(sem_t *sem);
</pre>

<p>
<b>Description:</b> This function is called to indicate that the
calling task is finished with the specified named semaphore, sem.
The sem_close() deallocates any system resources allocated by
the system for this named semaphore.
<p>
If the semaphore has not been removed with a call to sem_unlink(),
then sem_close() has no effect on the named semaphore. However,
when the named semaphore has been fully unlinked, the semaphore
will vanish when the last task closes it.
<p>
Care must be taken to avoid risking the deletion of a semaphore
that another calling task has already locked.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. Semaphore descriptor
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<ul>
<li>Care must be taken to avoid deletion of a semaphore that another task
has already locked.
<li>sem_close() must not be called with an un-named semaphore.
</ul>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="semunlink">2.5.5 sem_unlink</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_unlink(const char *name);
</pre>

<p>
<b>Description:</b> This function will remove the semaphore named by the
input name parameter.  If one or more tasks have the semaphore named by
name open when sem_unlink() is called, destruction of the semaphore will
be postponed until all references have been destroyed by calls to
sem_close().
<p>
<b>Input Parameters:</b>
<ul>
<li><code>name</code>. Semaphore name
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<ul>
<li>Care must be taken to avoid deletion of a semaphore that another task
has already locked.
<li>sem_unlink() must not be called with an un-named semaphore.
</ul>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the full POSIX implementation include:
<ul>
<li>Treatment of links/connections is highly simplified. It is
just a counting semaphore.
<li>Calls to sem_open() to re-create or re-connect to the semaphore may
refer to the same semaphore; POSIX specifies that a new semaphore with the
same name should be created after sem_unlink() is called.
</ul>

<H3><a name="semwait">2.5.6 sem_wait</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_wait(sem_t *sem);
</pre>

<p>
<b>Description:</b> This function attempts to lock the semaphore
referenced by sem. If the semaphore as already locked by another
task, the calling task will not return until it either successfully acquires
the lock or the call is interrupted by a signal.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. Semaphore descriptor.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) is unsuccessful
</ul>
<p>
If <code>sem_wait</code> returns -1 (<code>ERROR</code>) then the cause of the failure
will be indicated by the thread-specific <a href="#ErrnoAccess"><code>errno</code></a>.
The following lists the possible values for <a href="#ErrnoAccess"><code>errno</code></a>:
<p>
<ul>
<li><code>EINVAL</code>:  Indicates that the <code>sem</code> input parameter is
not valid.
<li><code>EINTR</code>:  Indicates that the wait was interrupt by a signal
received by this task.  In this case, the semaphore has not be acquired.
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="semtimedwait">2.5.7 sem_timedwait</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    #include &lt;time.h&gt;
    int sem_wait(sem_t *sem, const struct timespec *abstime);
</pre>

<p>
<b>Description:</b>
  This function will lock the semaphore referenced by sem as in the <code>sem_wait()</code> function.
  However, if the semaphore cannot be locked without waiting for another process or thread to unlock the semaphore by performing a <code>sem_post()</code> function, this wait will be terminated when the specified timeout expires.
</p>
  The timeout will expire when the absolute time specified by <code>abstime</code> passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abstime), or if the absolute time specified by abstime has already been passed at the time of the call.
  This function attempts to lock the semaphore referenced by <code>sem</code>.
  If the semaphore is already locked by another task, the calling task will not return until it either successfully acquires the lock or the call is interrupted by a signal.
<p>
<b>Input Parameters:</b>
<ul>
  <li>
    <code>sem</code>. Semaphore descriptor.
  </li>
  <li>
    <code>abstime</code>. The absolute time to wait until a timeout is declared.
  </li>
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) is unsuccessful
</ul>
<p>
If <code>sem_wait</code> returns -1 (<code>ERROR</code>) then the cause of the failure
will be indicated by the thread-specific <a href="#ErrnoAccess"><code>errno</code></a>.
The following lists the possible values for <a href="#ErrnoAccess"><code>errno</code></a>:
<p>
<ul>
</ul>
  <li>
    <code>EINVAL</code>:
      Indicates that the <code>sem</code> input parameter is not valid or the
      thread would have blocked, and the abstime parameter specified
      a nanoseconds field value less than zero or greater than or
      equal to 1000 million.
  </li>
  <li>
    <code>ETIMEDOUT</code>:
      The semaphore could not be locked before the specified timeout expired.
  </li>
  <li>
    <code>EDEADLK</code>:
      A deadlock condition was detected.
  </li>
  <li>
    <code>EINTR</code>:
      Indicates that the wait was interrupt by a signal received by this task.
      In this case, the semaphore has not be acquired.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Derived from IEEE Std 1003.1d-1999.
</p>

<H3><a name="semtrywait">2.5.8 sem_trywait</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_trywait(sem_t *sem);
</pre>

<p>
<b>Description:</b> This function locks the specified semaphore
only if the semaphore is currently not locked.  In any event, the call
returns without blocking.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. The semaphore descriptor
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) or -1 (<code>ERROR</code>) if unsuccessful
</ul>
If <code>sem_wait</code> returns -1 (<code>ERROR</code>) then the cause of the failure
will be indicated by the thread-specific <a href="#ErrnoAccess"><code>errno</code></a>.
The following lists the possible values for <a href="#ErrnoAccess"><code>errno</code></a>:
<p>
<ul>
<li><code>EINVAL</code>:  Indicates that the <code>sem</code> input parameter is
not valid.
<li><code>EAGAIN</code>:  Indicates that the semaphore was not acquired.
</ul>
<p>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sempost">2.5.9 sem_post</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_post(sem_t *sem);
</pre>

<p>
<b>Description:</b> When a task has finished with a semaphore,
it will call sem_post().  This function unlocks the semaphore referenced
by <code>sem</code> by performing the semaphore unlock operation.
<p>
If the semaphore value resulting from this operation is positive, then
no tasks were blocked waiting for the semaphore to become unlocked;
The semaphore value is simply incremented.
<p>
If the value of the semaphore resulting from this operation is zero, then
on of the tasks blocked waiting for the semaphore will be allowed to
return successfully from its call to <code>sem_wait()</code>.
<p>
<b>NOTE</b>: <code>sem_post()</code> may be called from an interrupt handler.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. Semaphore descriptor
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) or -1 (<code>ERROR</code>) if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>.
When called from an interrupt handler, it will appear as though the
interrupt task is the one that is performing the unlock.
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="semgetvalue">2.5.10 sem_getvalue</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;semaphore.h&gt;
    int sem_getvalue(sem_t *sem, int *sval);
</pre>

<p>
<b>Description:</b> This function updates the location referenced
by sval argument to have the value of the semaphore referenced
by sem without effecting the state of the semaphore. The updated
value represents the actual semaphore value that occurred at some
unspecified time during the call, but may not reflect the actual
value of the semaphore when it is returned to the calling task.
<p>
If sem is locked, the value return by sem_getvalue() will either
be zero or a negative number whose absolute value represents the
number of tasks waiting for the semaphore.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sem</code>. Semaphore descriptor
<li><code>sval</code>. Buffer by which the value is returned
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) or -1 (<code>ERROR</code>) if unsuccessful.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
</p>

<H3><a name="semgetprotocol">2.5.11 sem_getprotocol</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;nuttx/semaphore.h&gt;
    int sem_getprotocol(FAR const pthread_mutexattr_t *attr, FAR int *protocol);
</pre>
<p>
<b>Description:</b> Return the value of the semaphore protocol attribute.
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>attr</code>. A pointer to the semaphore to be queried</li>
  <li><code>protocol</code>. The user provided location in which to store the protocol value.  May be one of <code>SEM_PRIO_NONE</code>, or <code>SEM_PRIO_INHERIT</code>, <code>SEM_PRIO_PROTECT</code>.</li>
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
If successful, the <code>sem_getprotocol()</code> function will return zero (<code>OK</code>).
Otherwise, an -1 (<code>ERROR</code>) will be returned and the <code>errno</code> value will be set to indicate the nature of the error.
</p>
<p>
<b>Assumptions/Limitations:</b>
</p>
<p>
<b>POSIX Compatibility:</b> Non-standard NuttX interface.  Should not be used in portable code.  Analogous to <code>pthread_muxtexattr_getprotocol()</code>.
</p>

<H3><a name="semsetprotocol">2.5.12 sem_setprotocol</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;nuttx/semaphore.h&gt;
    int sem_setprotocol(FAR pthread_mutexattr_t *attr, int protocol);
</pre>
<p>
<b>Description:</b> Set semaphore protocol attribute.  See the paragraph <a href="#lockingvssignaling">Locking versus Signaling Semaphores</a> for some important information about the use of this interface.

</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>attr</code>. A pointer to the semaphore to be modified</li>
  <li><code>protocol</code>. The new protocol to use.  One of <code>SEM_PRIO_NONE</code>, or <code>SEM_PRIO_INHERIT</code>, <code>SEM_PRIO_PROTECT</code>.  <code>SEM_PRIO_INHERIT</code> is supported only if  <code>CONFIG_PRIORITY_INHERITANCE</code> is defined;  <code>SEM_PRIO_PROTECT</code> is not currently supported in any configuration.</li>
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
If successful, the <code>sem_getprotocol()</code> function will return zero (<code>OK</code>).
Otherwise, an -1 (<code>ERROR</code>) will be returned and the <code>errno</code> value will be set to indicate the nature of the error.
</p>
<p>
<b>Assumptions/Limitations:</b>
</p>
<p>
<b>POSIX Compatibility:</b> Non-standard NuttX interface.  Should not be used in portable code.  Analogous to <code>pthread_muxtexattr_setprotocol()</code>.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="ClocksNTimers"><h2>2.6 Clocks and Timers</h2></a>
  </td>
  </tr>
</table>

<ul>
  <li><a href="#clocksettime">2.6.1 clock_settime</a></li>
  <li><a href="#clockgettime">2.6.2 clock_gettime</a></li>
  <li><a href="#clockgetres">2.6.3 clock_getres</a></li>
  <li><a href="#mktime">2.6.4 mktime</a></li>
  <li><a href="#gmtime">2.6.5 gmtime</a></li>
  <li><a href="#localtime">2.6.6 localtime</a></li>
  <li><a href="#asctime">2.6.7 asctime</a></li>
  <li><a href="#ctime">2.6.8 ctime</a></li>
  <li><a href="#gmtimer">2.6.9 gmtime_r</a></li>
  <li><a href="#localtimer">2.6.10 localtime_r</a></li>
  <li><a href="#asctimer">2.6.11 asctime_r</a></li>
  <li><a href="#ctimer">2.6.12 ctime_r</a></li>
  <li><a href="#timercreate">2.6.13 timer_create</a></li>
  <li><a href="#timerdelete">2.6.14 timer_delete</a></li>
  <li><a href="#timersettime">2.6.15 timer_settime</a></li>
  <li><a href="#timergettime">2.6.16 timer_gettime</a></li>
  <li><a href="#timergetoverrun">2.6.17 timer_getoverrun</a></li>
  <li><a href="#gettimeofday">2.6.18 gettimeofday</a></li>
</ul>

<H3><a name="clocksettime">2.6.1 clock_settime</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int clock_settime(clockid_t clockid, const struct timespec *tp);
</pre>
<p>
  <b>Description:</b>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>parm</code>. </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>clock_settime()</code> function will return zero (<code>OK</code>).
  Otherwise, an non-zero error number will be returned to indicate the error:
</p>

<H3><a name="clockgettime">2.6.2 clock_gettime</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int clock_gettime(clockid_t clockid, struct timespec *tp);
</pre>
<p>
  <b>Description:</b>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>parm</code>. </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>clock_gettime()</code> function will return zero (<code>OK</code>).
  Otherwise, an non-zero error number will be returned to indicate the error:
</p>

<H3><a name="clockgetres">2.6.3 clock_getres</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int clock_getres(clockid_t clockid, struct timespec *res);
</pre>
<p>
  <b>Description:</b>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>parm</code>. </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>clock_getres()</code> function will return zero (<code>OK</code>).
  Otherwise, an non-zero error number will be returned to indicate the error:
</p>

<H3><a name="mktime">2.6.4 mktime</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    time_t mktime(struct tm *tp);
</pre>
<p>
  <b>Description:</b>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>parm</code>. </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>mktime()</code> function will return zero (<code>OK</code>).
  Otherwise, an non-zero error number will be returned to indicate the error:
</p>

<H3><a name="gmtime">2.6.5 gmtime</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    FAR struct tm *gmtime(FAR const time_t *timep);
</pre>
<p>
  <b>Description:</b>
  Represents GMT date/time in a type <code>struct tm</code>.
  This function is not re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timep</code>.
  Represents GMT calendar time.
  This is an absolute time  value representing the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
</li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>gmtime()</code> function will return the pointer to a statically defined instance of <code>struct tm</code>.
  Otherwise, a NULL will be returned to indicate the error:
</p>

<H3><a name="localtime">2.6.6 localtime</a></H3>
<ul><pre>
#include &lt;time.h&gt;
#ifdef CONFIG_LIBC_LOCALTIME
#  define localtime(c) gmtime(c)
#else
FAR struct tm *localtime(FAR const time_t *timep);
#endif
</pre></ul>
<p>
  <b>Description:</b>
  Represents local date/time in a type <code>struct tm</code>.
  This function is not re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timep</code>.
  Represents GMT calendar time.
  This is an absolute time  value representing the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
</ul>
<p>
  <b>Returned Value:</b>
  <b>Returned Value:</b>
  If successful, the <code>localtime()</code> function will return the pointer to a statically defined instance of <code>struct tm</code>.
  Otherwise, a NULL will be returned to indicate the error:
</p>

<h3><a name="asctime">2.6.7 asctime</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;time.h&gt;
FAR char *asctime(FAR const struct tm *tp);
</pre></ul>
<p>
  <b>Description:</b>
  <code>asctime()</code> convert the time provided in a <code>struct tm</code> to a string representation.
  <code>asctime()</code> is not re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>tp</code>.
    Pointer to the time to be converted.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>asctime()</code> function will return a pointer to a statically defined string holding the converted time.
  Otherwise, a NULL will be returned to indicate the error.
</p>

<h3><a name="ctime">2.6.8 ctime</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;time.h&gt;
FAR char *ctime(FAR const time_t *timep);
</pre></ul>
<p>
  <b>Description:</b>
  <code>ctime()</code> converts the time provided in seconds since the epoch to a string representation.
  <code>ctime()</code> is not re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timep</code>.
  The current time represented as seconds since the epoch.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>ctime()</code> function will return the pointer to the converted string.
  Otherwise, a NULL will be returned to indicate the error.
</p>

<h3><a name="gmtimer">2.6.9 gmtime_r</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    struct tm *gmtime_r(const time_t *timep, struct tm *result);
</pre>
<p>
  <b>Description:</b>
  Represents GMT date/time in a type <code>struct tm</code>.
  This function is re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timep</code>.
  Represents GMT calendar time.
  This is an absolute time  value representing the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
  <li><code>result</code>.
  A user-provided buffer to receive the converted time structure.
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>gmtime_r()</code> function will return the pointer, <code>result</code>,
  provided by the caller.
  Otherwise, a NULL will be returned to indicate the error:
</p>

<H3><a name="localtimer">2.6.10 localtime_r</a></H3>
<ul><pre>
#include &lt;time.h&gt;
#ifdef CONFIG_LIBC_LOCALTIME
#  define localtime_r(c,r) gmtime_r(c,r)
#else
FAR struct tm *localtime_r(FAR const time_t *timep, FAR struct tm *result);
#endif
</pre></ul>
<p>
  <b>Description:</b>
  Represents local date/time in a type <code>struct tm</code>.
  This function is re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timep</code>.
  Represents GMT calendar time.
  This is an absolute time  value representing the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
  <li><code>result</code>.
  A user-provided buffer to receive the converted time structure.
</ul>
<p>
  <b>Returned Value:</b>
  <b>Returned Value:</b>
  If successful, the <code>localtime_r()</code> function will return the pointer, <code>result</code>,
  provided by the caller.
  Otherwise, a NULL will be returned to indicate the error:
</p>

<h3><a name="asctimer">2.6.11 asctime_r</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;time.h&gt;
FAR char *asctime_r(FAR const struct tm *tp, FAR char *buf);
</pre></ul>
<p>
  <b>Description:</b>
  <code>asctime_r()</code> converts the time provided in a <code>struct tm</code> to a string representation.
  <code>asctime-r()</code> is re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>tp</code>.
    Pointer to the time to be converted.
  </li>
  <li><code>buf</code>.
    The user provider buffer. of size &gt;= 26 characters, to receive the converted time.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>asctime_r()</code> function will return the pointer, <code>buf</code>, provided by the caller.
  Otherwise, a NULL will be returned to indicate the error.
</p>

<h3><a name="ctimer">2.6.12 ctime_r</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;time.h&gt;
FAR char *ctime_r(FAR const time_t *timep, FAR char *buf);
</pre></ul>
<p>
  <b>Description:</b>
  <code>ctime_r()</code> converts the time provided in seconds since the epoch to a string representation.
  <code>ctime()</code> is re-entrant.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timep</code>.
    The current time represented as seconds since the epoch.
  </li>
  <li><code>buf</code>.
    The user provider buffer. of size &gt;= 26 characters, to receive the converted time.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>ctime_r()</code> function will return the pointer, <code>buf</code>, provided by the caller.
  Otherwise, a NULL will be returned to indicate the error.
</p>

<h3><a name="timercreate">2.6.13 timer_create</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int timer_create(clockid_t clockid, struct sigevent *evp, timer_t *timerid);
</pre>
<p>
  <b>Description:</b>
  The  <code>timer_create()</code> function creates per-thread timer using the specified
  clock, <code>clock_id</code>, as the timing base.
  The <code>timer_create()</code> function returns, in
  the location referenced by <code>timerid</code>, a timer ID of type timer_t used to identify
  the timer in timer requests.
  This timer ID is unique until the timer is deleted.
  The particular clock, <code>clock_id</code>, is defined in <code>&lt;time.h&gt;</code>.
  The timer whose ID is returned will be in a disarmed state upon return from
  <code>timer_create()</code>.
</p>
<p>
  The <code>evp</code> argument, if non-NULL, points to a <code>sigevent</code> structure.
  This structure is allocated by the called and defines the asynchronous notification to occur.
  If the <code>evp</code> argument is NULL, the effect is as if the <code>evp</code> argument pointed to
  a <code>sigevent</code> structure with the <code>sigev_notify</code> member having the value <code>SIGEV_SIGNAL</code>,
  the <code>sigev_signo</code> having a default signal number, and the <code>sigev_value</code> member
  having the value of the timer ID.
</p>
<p>
  Each implementation defines a set of clocks that can be used as timing bases
  for per-thread timers. All implementations will support a <code>clock_id</code> of
  <code>CLOCK_REALTIME</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>clockid</code>. Specifies the clock to use as the timing base.
    Must be <code>CLOCK_REALTIME</code>.</li>
  <li><code>evp</code>. Refers to a user allocated sigevent structure that defines the
    asynchronous notification.  evp may be NULL (see above).</li>
  <li><code>timerid</code>. The pre-thread timer created by the call to timer_create().</li>
</ul>
<p>
  <b>Returned Value:</b>
  If the call succeeds, <code>timer_create()</code> will return 0 (<code>OK</code>) and update the
  location referenced by <code>timerid</code> to a <code>timer_t</code>, which can be passed to the
  other per-thread timer calls.  If an error occurs, the function will return
  a value of -1 (<code>ERROR</code>) and set <a href="#ErrnoAccess"><code>errno</code></a> to indicate the error.
</p>
<ul>
  <li><code>EAGAIN</code>. The system lacks sufficient signal queuing resources to honor the
    request.</li>
  <li><code>EAGAIN</code>. The calling process has already created all of the timers it is
    allowed by this implementation.</li>
  <li><code>EINVAL</code>. The specified clock ID is not defined.</li>
  <li><code>ENOTSUP</code>. The implementation does not support the creation of a timer attached
    to the CPU-time clock that is specified by clock_id and associated with a
    thread different thread invoking timer_create().</li>
</ul>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:
</p>
<ul>
  <li>Only <code>CLOCK_REALTIME</code> is supported for the <code>clockid</code> argument.</li>
</ul>

<H3><a name="timerdelete">2.6.14 timer_delete</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int timer_delete(timer_t timerid);
</pre>
<p>
  <b>Description:</b>
  The <code>timer_delete()</code> function deletes the specified timer, <code>timerid</code>, previously
  created by the <code>timer_create()</code> function.
  If the timer is armed when <code>timer_delete()</code> is called, the timer will be automatically disarmed before
  removal.
  The disposition of pending signals for the deleted timer is unspecified.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>timerid</code>.
  The pre-thread timer, previously created by the call to timer_create(), to be deleted.</li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>timer_delete()</code> function will return zero (<code>OK</code>).
  Otherwise, the function will return a value of -1 (<code>ERROR</code>) and set
  <a href="#ErrnoAccess"><code>errno</code></a> to indicate the error:
</p>
<ul>
  <li><code>EINVAL</code>. The timer specified timerid is not valid.</li>
</ul>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<H3><a name="timersettime">2.6.15 timer_settime</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int timer_settime(timer_t timerid, int flags, const struct itimerspec *value,
                      struct itimerspec *ovalue);
</pre>
<p>
  <b>Description:</b>
  The <code>timer_settime()</code> function sets the time until the next expiration of the
  timer specified by <code>timerid</code> from the <code>it_value</code> member of the value argument
  and arm the timer if the <code>it_value</code> member of value is non-zero. If the
  specified timer was already armed when <code>timer_settime()</code> is called, this call
  will reset the time until next expiration to the value specified. If the
  <code>it_value</code> member of value is zero, the timer will be disarmed. The effect
  of disarming or resetting a timer with pending expiration notifications is
  unspecified.
</p>
<p>
  If the flag <code>TIMER_ABSTIME</code> is not set in the argument flags, <code>timer_settime()</code>
  will behave as if the time until next expiration is set to be equal to the
  interval specified by the <code>it_value</code> member of value. That is, the timer will
  expire in <code>it_value</code> nanoseconds from when the call is made. If the flag
  <code>TIMER_ABSTIME</code> is set in the argument flags, <code>timer_settime()</code> will behave as
  if the time until next expiration is set to be equal to the difference between
  the absolute time specified by the <code>it_value</code> member of value and the current
  value of the clock associated with <code>timerid</code>.  That is, the timer will expire
  when the clock reaches the value specified by the <code>it_value</code> member of value.
  If the specified time has already passed, the function will succeed and the
  expiration notification will be made.
</p>
<p>
  The reload value of the timer will be set to the value specified by the
  <code>it_interval</code> member of value.  When a timer is armed with a non-zero
  <code>it_interval</code>, a periodic (or repetitive) timer is specified.
</p>
<p>
  Time values that are between two consecutive non-negative integer multiples
  of the resolution of the specified timer will be rounded up to the larger
  multiple of the resolution. Quantization error will not cause the timer to
  expire earlier than the rounded time value.
</p>
<p>
  If the argument <code>ovalue</code> is not NULL, the t<code>imer_settime()</code> function will store,
  in the location referenced by <code>ovalue</code>, a value representing the previous
  amount of time before the timer would have expired, or zero if the timer was
  disarmed, together with the previous timer reload value. Timers will not
  expire before their scheduled time.
</p>
  <b>NOTE:</b>At present, the <code>ovalue</code> argument is ignored.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
 <li><code>timerid</code>. The pre-thread timer, previously created by the call to timer_create(), to be be set.</li>
 <li><code>flags</code>. Specify characteristics of the timer (see above)</li>
 <li><code>value</code>. Specifies the timer value to set</li>
 <li><code>ovalue</code>. A location in which to return the time remaining from the previous timer setting (ignored).</li>
</ul>
<p>
  <b>Returned Value:</b>
  If the timer_gettime() succeeds, a value of 0 (<code>OK</code>) will be returned.
  If an error occurs, the value -1 (<code>ERROR</code>) will be returned, and
  <a href="#ErrnoAccess"><code>errno</code></a> set to indicate the error.
</p>
<ul>
  <li><code>EINVAL</code>. The timerid argument does not correspond to an ID returned by timer_create() but not yet deleted by timer_delete().</li>
  <li><code>EINVAL</code>. A value structure specified a nanosecond value less than zero or greater than or equal to 1000 million,
    and the it_value member of that structure did not specify zero seconds and nanoseconds.</li>
</ul>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:
</p>
<ul>
  <li>The <code>ovalue</code> argument is ignored.</li>
</ul>

<H3><a name="timergettime">2.6.16 timer_gettime</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int timer_gettime(timer_t timerid, struct itimerspec *value);
</pre>
<p>
  <b>Description:</b>
  The <code>timer_gettime()</code> function will store the amount of time until the
  specified timer, <code>timerid</code>, expires and the reload value of the timer into the
  space pointed to by the <code>value</code> argument. The <code>it_value</code> member of this structure
  will contain the amount of time before the timer expires, or zero if the timer
  is disarmed. This value is returned as the interval until timer expiration,
  even if the timer was armed with absolute time. The <code>it_interval</code> member of
  <code>value</code> will contain the reload value last set by <code>timer_settime()</code>.
</p>
<p>
   Due to the asynchronous operation of this function, the time reported
   by this function could be significantly more than that actual time
   remaining on the timer at any time.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
 <li><code>timerid</code>. Specifies pre-thread timer, previously created by the call to
 t<code>imer_create()</code>, whose remaining count will be returned.</li>
</ul>
<p>
  <b>Returned Value:</b>
  If successful, the <code>timer_gettime()</code> function will return zero (<code>OK</code>).
  Otherwise, an non-zero error number will be returned to indicate the error:
</p>
<ul>
  <li><code>EINVAL</code>.
  The <code>timerid</code> argument does not correspond to an ID returned by
  <code>timer_create()</code> but not yet deleted by <code>timer_delete()</code>.</li>
</ul>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<H3><a name="timergetoverrun">2.6.17 timer_getoverrun</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;time.h&gt;
    int timer_getoverrun(timer_t timerid);
</pre>
<p>
  <b>Description:</b>
  Only a single signal will be queued to the process for a given timer at any
  point in time.  When a timer for which a signal is still pending expires, no
  signal will be queued, and a timer overrun will occur. When a timer
  expiration signal is delivered to or accepted by a process, if the
  implementation  supports  the  <i>Realtime Signals Extension</i>, the
  <code>timer_getoverrun()</code> function will return the timer expiration overrun count for
  the specified timer. The overrun count returned contains the number of extra
  timer expirations that occurred between the time the signal was generated
  (queued) and when it was delivered or accepted, up to but not including an
  implementation-defined  maximum of <code>DELAYTIMER_MAX</code>. If the number of such
  extra expirations is greater than or equal to <code>DELAYTIMER_MAX</code>, then the
  overrun count will be set to <code>DELAYTIMER_MAX</code>. The value returned by
  <code>timer_getoverrun()</code> will apply to the most recent expiration signal delivery
  or acceptance for the timer.  If no expiration signal has been delivered
  for the timer, or if the <i>Realtime Signals Extension</i> is not supported, the
  return value of <code>timer_getoverrun()</code> is unspecified.
</p>
<p>
  <b>NOTE:</b>  This interface is not currently implemented in NuttX.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
 <li><code>timerid</code>. Specifies pre-thread timer, previously created by the call to
 <code>timer_create()</code>, whose overrun count will be returned.</li>
</ul>
<p>
  <b>Returned Value:</b>
  If the <code>timer_getoverrun()</code> function succeeds, it will return the timer
  expiration overrun count as explained above. <code>timer_getoverrun()</code> will fail if:
</p>
<ul>
  <li><code>EINVAL</code>.
  The <code>timerid</code> argument does not correspond to an ID returned by
  <code>timer_create()</code> but not yet deleted by <code>timer_delete()</code>.</li>
</ul>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:
</p>
<ul>
  <li>This interface is not currently implemented by NuttX.</li>
</ul>

<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
</p>

<h3><a name="gettimeofday">2.6.18 gettimeofday</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;sys/time.h&gt;
    int gettimeofday(struct timeval *tp, void *tzp);
</pre>
<p>
  <b>Description:</b>
  This implementation of <code>gettimeofday()</code> is simply a thin wrapper around
  <a href="#clockgettime"><code>clock_gettime()</code></a>.
  It simply calls <code>clock_gettime()</code> using the <code>CLOCK_REALTIME</code> timer and
  converts the result to the required <code>struct timeval</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
 <li><code>tp</code>. The current time will be returned to this user provided location.</li>
 <li><code>tzp</code>. A reference to the timezone -- <i>IGNORED</i>.</li>
</ul>
<p>
  <b>Returned Value:</b>
  See <a href="#clockgettime"><code>clock_gettime()</code></a>.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Signals"><h2>2.7 Signal Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  <b>Tasks and Signals</b>.
  NuttX provides signal interfaces for tasks and pthreads.
  Signals are used toalter the flow control of tasks by communicating asynchronous events within or between task contexts.
  Any task or interrupt handler can post (or send) a signal to a particular task using its task ID.
  The task being signaled will execute task-specified signal handler function the next time that the task has priority.
  The signal handler is a user-supplied function that is bound to a specific signal and performs whatever actions are necessary whenever the signal is received.
</p>
<p>
  By default, here are no predefined actions for any signal.
  The default action for all signals (i.e., when no signal handler has been supplied by the user) is to ignore the signal.
  In this sense, all NuttX are <i>real time</i> signals by default.
  If the configuration option <code>CONFIG_SIG_DEFAULT=y</code> is included, some signals will perform their default actions dependent upon addition configuration settings as summarized in the following table:
</p>
<p><ul><table>
  <tr>
    <th>Signal</th>
    <th>Action</th>
    <th>Additional Configuration</th>
  </tr>
  <tr>
    <td><code>SIGUSR1</code></td>
    <td>Abnormal Termination</td>
    <td><code>CONFIG_SIG_SIGUSR1_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGUSR2</code></td>
    <td>Abnormal Termination</td>
    <td><code>CONFIG_SIG_SIGUSR2_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGALRM</code></td>
    <td>Abnormal Termination</td>
    <td><code>CONFIG_SIG_SIGALRM_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGPOLL</code></td>
    <td>Abnormal Termination</td>
    <td><code>CONFIG_SIG_SIGPOLL_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGSTOP</code></td>
    <td>Suspend task</td>
    <td><code>CONFIG_SIG_SIGSTOP_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGSTP</code></td>
    <td>Suspend task</td>
    <td><code>CONFIG_SIG_SIGSTOP_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGCONT</code></td>
    <td>Resume task</td>
    <td><code>CONFIG_SIG_SIGSTOP_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGINT</code></td>
    <td>Abnormal Termination</td>
    <td><code>CONFIG_SIG_SIGKILL_ACTION</code></td>
  </tr>
  <tr>
    <td><code>SIGKILL</code></td>
    <td>Abnormal Termination</td>
    <td><code>CONFIG_SIG_SIGKILL_ACTION</code></td>
  </tr>
</table></ul></p>
<p>
  Tasks may also suspend themselves and wait until a signal is received.
</p>
<p>
  <b>Tasks Groups</b>.
  NuttX supports both tasks and pthreads.
  The primary difference between tasks and pthreads is the tasks are much more independent.
  Tasks can create pthreads and those pthreads will share the resources of the task.
  The main task and its children pthreads together are referred as a <i>task group</i>.
  A task group is used in NuttX to emulate a POSIX <i>process</i>.
</p>
<blockquote><small>
  <b>NOTE:</b>
  Behavior of features related to <i>task group</i>s depend of NuttX configuration settings.
  See the <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:nxtasking">NuttX Threading Wiki</a> page and the <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:tasksnthreads">Tasks vs. Threads FAQ</a> for additional information on tasks and threads in NuttX.
</small></blockquote>
<p>
  <b>Signaling Multi-threaded Task Groups</b>.
  The behavior of signals in the multi-thread task group is complex.
  NuttX emulates a process model with task groups and follows the POSIX rules for signaling behavior.
  Normally when you signal the task group you would signal using the task ID of the main task that created the group (in practice, a different task should not know the IDs of the internal threads created within the task group); that ID is remembered by the task group (even if the main task thread exits).
</p>
<p>
  Here are some of the things that should happen when you signal a multi-threaded task group:
</p>
<ul>
  <li>
    If a task group receives a signal then one and only one indeterminate thread in the task group which is not blocking the signal will receive the signal.
  </li>
  <li>
    If a task group receives a signal and more than one thread is waiting on that signal, then one and only one indeterminate thread out of that waiting group will receive the signal.
  </li>
</ul>
<p>
  You can mask out that signal using ''sigprocmask()'' (or ''pthread_sigmask()'').
  That signal will then be effectively disabled and will never be received in those threads that have the signal masked.
  On creation of a new thread, the new thread will inherit the signal mask of the parent thread that created it.
  So you if block signal signals on one thread then create new threads, those signals will also be blocked in the new threads as well.
</p>
<p>
  You can control which thread receives the signal by controlling the signal mask.
  You can, for example, create a single thread whose sole purpose it is to catch a particular signal and respond to it:  Simply block the signal in the main task; then the signal will be blocked in all of the pthreads in the group too.  In the one "signal processing" pthread, enable the blocked signal. This thread will then be only thread that will receive the signal.
</p>
<p>
  <b>Signal Interfaces</b>.
  The following signal handling interfaces are provided by NuttX:
</p>
<ul>
  <li><a href="#sigemptyset">2.7.1 sigemptyset</a></li>
  <li><a href="#sigfillset">2.7.2 sigfillset</a></li>
  <li><a href="#sigaddset">2.7.3 sigaddset</a></li>
  <li><a href="#sigdelset">2.7.4 sigdelset</a></li>
  <li><a href="#sigismember">2.7.5 sigismember</a></li>
  <li><a href="#sigaction">2.7.6 sigaction</a></li>
  <li><a href="#sigignore">2.7.7 sigignore</a></li>
  <li><a href="#sigset">2.7.8 sigset</a></li>
  <li><a href="#sigprocmask">2.7.9 sigprocmask</a></li>
  <li><a href="#sighold">2.7.10 sighold</a></li>
  <li><a href="#sigrelse">2.7.11 sigrelse</a></li>
  <li><a href="#sigpending">2.7.12 sigpending</a></li>
  <li><a href="#sigsuspend">2.7.13 sigsuspend</a></li>
  <li><a href="#sigpause">2.7.14 sigpause</a></li>
  <li><a href="#sigwaitinfo">2.7.15 sigwaitinfo</a></li>
  <li><a href="#sigtimedwait">2.7.16 sigtimedwait</a></li>
  <li><a href="#sigqueue">2.7.17 sigqueue</a></li>
  <li><a href="#kill">2.7.18 kill</a></li>
  <li><a href="#pause">2.7.19 pause</a></li>
</ul>

<H3><a name="sigemptyset">2.7.1 sigemptyset</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigemptyset(sigset_t *set);
</pre>

<p>
<b>Description:</b> This function initializes the signal set specified
by set such that all signals are excluded.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>. Signal set to initialize.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if the signal set cannot be initialized.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sigfillset">2.7.2 sigfillset</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigfillset(sigset_t *set);
</pre>

<p>
<b>Description:</b> This function initializes the signal set specified
by set such that all signals are included.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>. Signal set to initialize
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if the signal set cannot be initialized.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sigaddset">2.7.3 sigaddset</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigaddset(sigset_t *set, int signo);
</pre>

<p>
<b>Description:</b> This function adds the signal specified by
signo to the signal set specified by set.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>. Signal set to add signal to
<li><code>signo</code>. Signal to add
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if the signal number is invalid.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sigdelset">2.7.4 sigdelset</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigdelset(sigset_t *set, int signo);
</pre>

<p>
<b>Description:</b> This function deletes the signal specified
by signo from the signal set specified by set.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>. Signal set to delete the signal from
<li><code>signo</code>. Signal to delete
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if the signal number is invalid.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sigismember">2.7.5 sigismember</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int  sigismember(const sigset_t *set, int signo);
</pre>

<p>
<b>Description:</b> This function tests whether the signal specified
by signo is a member of the set specified by set.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>. Signal set to test
<li><code>signo</code>. Signal to test for
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>1 (TRUE), if the specified signal is a member of the set,
<li>0 (OK or FALSE), if it is not, or
<li>-1 (<code>ERROR</code>) if the signal number is invalid.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sigaction">2.7.6 sigaction</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigaction(int signo, const struct sigaction *act,
                  struct sigaction *oact);
</pre>

<p>
<b>Description:</b> This function allows the calling task to
examine and/or specify the action to be associated with a specific
signal.
<p>
The structure sigaction, used to describe an action to be taken, is defined
to include the following members:
<ul>
<li><code>sa_u.sa_handler</code>.  A pointer to a signal-catching function.
<li><code>sa_u.sa_sigaction</code>.  An alternative form for the signal catching
function.
<li><code>sa_mask</code>.  Additional set of signals to be blocked during
execution of the signal-catching function.
<li><code>sa_flags</code>:  Special flags to affect behavior of a signal.
</ul>
<p>
If the argument act is not NULL, it points to a structure specifying the
action to be associated with the specified signal.  If the argument oact
is not NULL, the action previously associated with the signal is stored
in the location pointed to by the argument oact.  If the argument act is
NULL, signal handling is unchanged by this function call; thus, the call
can be used to inquire about the current handling of a given signal.
<p>
When a signal is caught by a signal-catching function installed by the
sigaction() function, a new signal mask is calculated and installed for
the duration of the signal-catching function.  This mask is formed by taking
the union of the current signal mask and the value of the sa_mask for the
signal being delivered, and then including the signal being delivered.  If
and when the signal handler returns, the original signal mask is restored.
<p>
Signal catching functions execute in the same address environment as the
task that called sigaction() to install the signal-catching function.
<p>
Once an action is installed for a specific signal, it remains installed
until another action is explicitly requested by another call to
sigaction().
<p>
<b>Input Parameters:</b>
<ul>
<li><code>sig</code>. Signal of interest
<li><code>act</code>. Location of new handler
<li><code>oact</code>. Location to store old handler
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if the signal number is invalid.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the POSIX implementation include:
<ul>
  <li>
    There are no default actions so the special value <code>SIG_DFL</code> is treated like <code>SIG_IGN</code>.
  </li>
  <li>
    All <code>sa_flags</code> in struct sigaction of act input are ignored (all treated like <code>SA_SIGINFO</code>).
    The one exception is if <code>CONFIG_SCHED_CHILD_STATUS</code> is defined;
    then <code>SA_NOCLDWAIT</code> is supported but only for <code>SIGCHLD</code>.
  </li>
</ul>

<H3><a name="sigignore">2.7.7 sigignore</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigignore(int signo);
</pre>

<p>
<b>Description:</b> The sigignore() function will set the disposition of <code>signo</code> to <code>SIG_IGN</code>.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>signo</code>.  The signal number to act on
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>Zero is returned upon successful completion, otherwise -1 (<code>ERROR</code>) is returned with the <code<>errno</code> set appropriately.  The <code>errno</code> value of <code>EINVAL</code>, for example, would indicate that <code>signo</code> argument is not a valid signal number.
</ul>

<H3><a name="sigset">2.7.8 sigset</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    void (*sigset(int signo, void (*disp)(int)))(int);
</pre>

<p>
<b>Description:</b>
  The sigset() function will modify signal dispositions.
  The <code>signo</code> argument specifies the signal.
  The <code>disp</code> argument specifies the signal's disposition, which may be <code>SIG_DFL</code>, <code>SIG_IGN</code>, or the address of a signal handler.
  If <code>disp</code> is the address of a signal handler, the system will add <code>signo</code> to the calling process' signal mask before executing the signal handler; when the signal handler returns, the system will restore the calling process' signal mask to its state prior to the delivery of the signal.
  <code>signo</code> will be removed from the calling process' signal mask.
</p>
<p>
  NOTE:  The value <code>SIG_HOLD</code> for <code>disp</code> is not currently supported.
</p>
<p>
<b>Input Parameters:</b>
<ul>
<li><code>signo</code>.  The signal number to operate on
<li><code>disp</code>.  The new disposition of the signal
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>
Upon successful completion, <code>sigset()</code> will the previous disposition of the signal.
Otherwise, <code>SIG_ERR</code> will be returned and <code>errno</code> set to indicate the error.
</ul>

<H3><a name="sigprocmask">2.7.9 sigprocmask</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigprocmask(int how, const sigset_t *set, sigset_t *oset);
</pre>

<p>
<b>Description:</b> This function allows the calling task to
examine and/or change its signal mask. If the set is not NULL,
then it points to a set of signals to be used to change the currently
blocked set. The value of how indicates the manner in which the
set is changed.
<p>
If there are any pending unblocked signals after the call to sigprocmask(),
those signals will be delivered before sigprocmask() returns.
<p>
If sigprocmask() fails, the signal mask of the task is not changed.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>how</code>. How the signal mast will be changed:
<ul>
<li><code>SIG_BLOCK</code>. The resulting set is the union of the
current set and the signal set pointed to by the <code>set</code> input parameter.
<li><code>SIG_UNBLOCK</code>. The resulting set is the intersection
of the current set and the complement of the signal set pointed
to by the <code>set</code> input parameter.
<li><code>SIG_SETMASK</code>. The resulting set is the signal set
pointed to by the <code>set</code> input parameter.
</ul>

<li><code>set</code>. Location of the new signal mask
<li><code>oset</code>. Location to store the old signal mask
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>), or -1 (<code>ERROR</code>) if how is invalid.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sighold">2.7.10 sighold</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sighold(int signo);
</pre>

<p>
<b>Description:</b> The sighold() function will add <code>signo</code> to the calling process' signal mask
</p>
<p>
<b>Input Parameters:</b>
<ul>
<li><code>signo</code>.  Identifies the signal to be blocked.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>Zero is returned upon successful completion, otherwise -1 (<code>ERROR</code>) is returned with the <code<>errno</code> set appropriately.  The <code>errno</code> value of <code>EINVAL</code>, for example, would indicate that <code>signo</code> argument is not a valid signal number.
</ul>

<H3><a name="sigrelse">2.7.11 sigrelse</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigrelse(int signo);
</pre>

<p>
<b>Description:</b> The sighold() function will remove <code>signo</code> from the calling process' signal mask
</p>
<p>
<b>Input Parameters:</b>
<ul>
<li><code>signo</code>.  Identifies the signal to be unblocked.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>Zero is returned upon successful completion, otherwise -1 (<code>ERROR</code>) is returned with the <code<>errno</code> set appropriately.  The <code>errno</code> value of <code>EINVAL</code>, for example, would indicate that <code>signo</code> argument is not a valid signal number.
</ul>

<H3><a name="sigpending">2.7.12 sigpending</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigpending(sigset_t *set);
</pre>

<p>
<b>Description:</b> This function stores the returns the set of
signals that are blocked for delivery and that are pending for
the calling task in the space pointed to by set.
<p>
If the task receiving a signal has the signal blocked via its
sigprocmask, the signal will pend until it is unmasked.  Only one pending
signal (for a given signo) is retained by the system.  This is consistent
with POSIX which states:  &quot;If a subsequent occurrence of a pending
signal is generated, it is implementation defined as to whether the signal
is delivered more than once.&quot;
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>. The location to return the pending signal set.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>0 (<code>OK</code>) or -1 (<code>ERROR</code>)
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="sigsuspend">2.7.13 sigsuspend</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigsuspend(const sigset_t *set);
</pre>

<p>
<b>Description:</b> The sigsuspend() function replaces the signal mask
with the set of signals pointed to by the argument set and then suspends
the task until delivery of a signal to the task.
<p>
If the effect of the set argument is to unblock a pending signal, then
no wait is performed.
<p>
The original signal mask is restored when sigsuspend() returns.
<p>
Waiting for an empty signal set stops a task without freeing any
resources (a very bad idea).
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>.  The value of the signal <b>mask</b> to use while
suspended.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>-1 (<code>ERROR</code>) always
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the POSIX specification include:
<ul>
<li>POSIX does not indicate that the original signal mask is restored.
<li>POSIX states that sigsuspend() &quot;suspends the task until
delivery of a signal whose action is either to execute a signal-catching
function or to terminate the task.&quot;  Only delivery of the signal
is required in the present implementation (even if the signal is ignored).
</ul>

<H3><a name="sigpause">2.7.14 sigpause</a></H3>
<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigpause(int signo);
</pre>

<p>
<b>Description:</b> The <code>sigpause()</code>) function will remove <code>signo</code>) from the calling process' signal mask and suspend the calling process until a signal is received.
The <code>sigpause()</code>) function will restore the process' signal mask to its original state before returning.
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>.  Identifies the signal to be unblocked while waiting.
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li><code>sigpause</code> always returns -1 (<code>ERROR</code>).  On a successful wait for a signal, the <code>errno</code> will be set to <code>EINTR</code>.
</ul>

<H3><a name="sigwaitinfo">2.7.15 sigwaitinfo</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigwaitinfo(const sigset_t *set, struct siginfo *info);
</pre>

<p>
<b>Description:</b> This function is equivalent to sigtimedwait()
with a NULL timeout parameter. (see below).
<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>.  The set of pending signals to wait for.
<li><code>info</code>. The returned signal values
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>Signal number that cause the wait to be terminated, otherwise
-1 (<code>ERROR</code>) is returned.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX interface of the same name.

<H3><a name="sigtimedwait">2.7.16 sigtimedwait</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigtimedwait(const sigset_t *set, struct siginfo *info,
                     const struct timespec *timeout);
</pre>

<p>
<b>Description:</b> This function selects the pending signal set
specified by the argument set. If multiple signals are pending in set,
it will remove and return the lowest numbered one. If no signals in set
are pending at the time of the call, the calling task will be suspended
until one of the signals in set becomes pending OR until the task
interrupted by an unblocked signal OR until the time interval specified by
timeout (if any), has expired. If timeout is NULL, then the timeout interval
is forever.
<p>
If the info argument is non-NULL, the selected signal number is
stored in the si_signo member and the cause of the signal is store
in the si_code member. The content of si_value is only meaningful
if the signal was generated by sigqueue(). The following values
for si_code are defined in signal.h:
<ul>
  <li><code>SI_USER</code>. Signal sent from kill, raise, or abort
  <li><code>SI_QUEUE</code>. Signal sent from sigqueue
  <li><code>SI_TIMER</code>. Signal is result of timer expiration
  <li><code>SI_ASYNCIO</code>. Signal is the result of asynchronous IO completion
  <li><code>SI_MESGQ</code>. Signal generated by arrival of a message on an empty message queue.
</ul>

<p>
<b>Input Parameters:</b>
<ul>
<li><code>set</code>.  The set of pending signals to wait for.
<li><code>info</code>. The returned signal values
<li><code>timeout</code>. The amount of time to wait
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>Signal number that cause the wait to be terminated, otherwise
-1 (<code>ERROR</code>) is returned.
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the POSIX interface include:
<ul>
<li>Values for si_codes differ
<li>No mechanism to return cause of ERROR. (It can be inferred
from si_code in a non-standard way).
<li>POSIX states that &quot;If no signal is pending at the time of the
call, the calling task will be suspended until one or more signals
in set become pending or until it is interrupted by an unblocked,
<i>caught</i> signal.&quot;  The present implementation does not require
that the unblocked signal be caught; the task will be resumed even if
the unblocked signal is ignored.
</ul>

<H3><a name="sigqueue">2.7.17 sigqueue</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
    #include &lt;signal.h&gt;
    int sigqueue (int tid, int signo, union sigval value);
</pre>

<p>
<b>Description:</b> This function sends the signal specified by
signo with the signal parameter value to the task specified
by tid.
<p>
If the receiving task has the signal blocked via its sigprocmask,
the signal will pend until it is unmasked.  Only one pending signal
(for a given signo) is retained by the system.  This is consistent with
POSIX which states:  &quot;If a subsequent occurrence of a pending signal
is generated, it is implementation defined as to whether the signal
is delivered more than once.&quot;
<p>
<b>Input Parameters:</b>
<ul>
<li><code>tid</code>. ID of the task to receive signal
<li><code>signo</code>. Signal number
<li><code>value</code>. Value to pass to task with signal
</ul>

<p>
<b>Returned Value:</b>
<ul>
<li>
  On  success (at least one signal was sent), zero (<code>OK</code>) is returned.
  On error, -1 (<code>ERROR</code>) is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately.
  <ul>
    <li><code>EGAIN</code>. The limit of signals which may be queued has been reached.</li>
    <li><code>EINVAL</code>. signo was invalid.</li>
    <li><code>EPERM</code>.  The task does not have permission to send the signal to the receiving process.</li>
    <li><code>ESRCH</code>. No process has a PID matching pid.</li>
  </ul>
</ul>

<p>
<b>Assumptions/Limitations:</b>
<p>
<b>  POSIX  Compatibility:</b> Comparable to the POSIX
interface of the same name.
Differences from the POSIX interface include:
<ul>
<li>Default action is to ignore signals.
<li>Signals are processed one at a time in order
<li>POSIX states that, &quot;If signo is zero (the null signal), error
checking will be performed but no signal is actually sent.&quot;
There is no null signal in the present implementation; a zero signal will
be sent.
</ul>

<H3><a name="kill">2.7.18 kill</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
   #include &lt;sys/types.h&gt;
   #include &lt;signal.h&gt;
   int kill(pid_t pid, int sig);
</pre>

<p>
<b>Description:</b>
  The kill() system call can be used to send any signal to
  any task.
</p>
<p>
  If the receiving task has the signal blocked via its sigprocmask,
  the signal will pend until it is unmasked.  Only one pending signal
  (for a given signo) is retained by the system.  This is consistent with
  POSIX which states:  &quot;If a subsequent occurrence of a pending signal
  is generated, it is implementation defined as to whether the signal
  is delivered more than once.&quot;
</p>
<p>
<b>Input Parameters:</b>
<ul>
<li><code>pid</code>. The id of the task to receive the signal.
  The POSIX <code>kill()</code> specification encodes process group
  information as zero and negative pid values.
  Only positive, non-zero values of pid are supported by this
  implementation. ID of the task to receive signal
<li><code>signo</code>. The signal number to send.
  If signo is zero, no signal is sent, but all error checking is performed.
</ul>

<p>
  <b>Returned Value:</b>
  <ul>
    <li>OK or ERROR
  </ul>
</p>

<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
    Comparable to the POSIX interface of the same name.
    Differences from the POSIX interface include:
</p>
<ul>
  <li>Default action is to ignore signals.</li>
  <li>Signals are processed one at a time in order </li>
  <li>Sending of signals to 'process groups' is not supported in NuttX.</li>
</ul>

<H3><a name="pause">2.7.19 pause</a></H3>

<p>
<b>Function Prototype:</b>
<pre>
   #include &lt;unistd.h&gt;
   int pause(void);
</pre>

<p>
<b>Description:</b>
  The <code>pause()</code> function will suspend the calling thread until delivery of a non-blocked signal.
</p>
<b>Input Parameters:</b>
<ul>
<li><i>None</i>
</ul>

<p>
  <b>Returned Value:</b>
 Since <code>pause()</code> suspends thread execution indefinitely unless interrupted a signal, there is no successful completion return value.
 A value of -1 (<code>ERROR</code> will always be returned and errno set to indicate the error (<code>EINTR</code>).
</p>

<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX  Compatibility:</b>
    In the POSIX description of this function is the <code>pause()</code> function will suspend the calling thread until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process.
    This implementation only waits for any non-blocked signal to be received.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Pthread"><h2>2.8 Pthread Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  NuttX does not support <i>processes</i> in the way that, say, Linux does.
  NuttX only supports simple threads or tasks running within the same address space.
  However, NuttX does support the concept of a <i>task group</i>.
  A task group is the functional analog of a process:
  It is a group that consists of the main task thread and of all of the pthreads created by the main thread or any of the other pthreads within the task group.
  Members of a task group share certain resources such as environment variables, file descriptors, <code>FILE</code> streams, sockets, pthread keys and open message queues.
</p>
<blockquote><small>
  <b>NOTE:</b>
  Behavior of features related to <i>task group</i>s depend of NuttX configuration settings.
  See the <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:nxtasking">NuttX Threading Wiki</a> page and the <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:tasksnthreads">Tasks vs. Threads FAQ</a> for additional information on tasks and threads in NuttX.
</small></blockquote>
<p>
  The following pthread interfaces are supported in some form by NuttX:
</p>
<ul>
  <p>
    <b>pthread control interfaces</b>.
    Interfaces that allow you to create and manage pthreads.
  </p>
  <ul>
    <li><a href="#pthreadattrinit">2.8.1 pthread_attr_init</a></li>
    <li><a href="#pthreadattrdestroy">2.8.2 pthread_attr_destroy</a></li>
    <li><a href="#pthreadattrsetschedpolity">2.8.3 pthread_attr_setschedpolicy</a></li>
    <li><a href="#pthreadattrgetschedpolicy">2.8.4 pthread_attr_getschedpolicy</a></li>
    <li><a href="#pthreadattrsetschedparam">2.8.5 pthread_attr_setschedparam</a></li>
    <li><a href="#pthreadattrgetschedparam">2.8.6 pthread_attr_getschedparam</a></li>
    <li><a href="#pthreadattrsetinheritsched">2.8.7 pthread_attr_setinheritsched</a></li>
    <li><a href="#pthreadattrgetinheritsched">2.8.8 pthread_attr_getinheritsched</a></li>
    <li><a href="#pthreadattrsetstacksize">2.8.9 pthread_attr_setstacksize</a></li>
    <li><a href="#pthreadattrgetstacksize">2.8.10 pthread_attr_getstacksize</a></li>
    <li><a href="#pthreadcreate">2.8.11 pthread_create</a></li>
    <li><a href="#pthreaddetach">2.8.12 pthread_detach</a></li>
    <li><a href="#pthreadexit">2.8.13 pthread_exit</a></li>
    <li><a href="#pthreadcancel">2.8.14 pthread_cancel</a></li>
    <li><a href="#pthreadsetcancelstate">2.8.15 pthread_setcancelstate</a></li>
    <li><a href="#pthreadsetcanceltype">2.8.16 pthread_setcanceltype</a></li>
    <li><a href="#pthreadtestcancel">2.8.17 pthread_testcancel</a></li>
    <li><a href="#pthreadcleauppop">2.8.18 pthread_cleanup_pop</a></li>
    <li><a href="#pthreadcleauppush">2.8.19 pthread_cleanup_push</a></li>
    <li><a href="#pthreadjoin">2.8.20 pthread_join</a></li>
    <li><a href="#pthreadyield">2.8.21 pthread_yield</a></li>
    <li><a href="#pthreadself">2.8.22 pthread_self</a></li>
    <li><a href="#pthreadgetschedparam">2.8.23 pthread_getschedparam</a></li>
    <li><a href="#pthreadsetschedparam">2.8.24 pthread_setschedparam</a></li>
  </ul>
  <p>
    <b>Thread Specific Data</b>.
    These interfaces can be used to create pthread <i>keys</i> and then to access thread-specific data using these keys.
    Each <i>task group</i> has its own set of pthread keys.
    NOTES: (1) pthread keys create in one <i>task group</i> are not accessible in other task groups.
    (2) The main task thread does not have thread-specific data.
  </p>
  <ul>
    <li><a href="#pthreadkeycreate">2.8.25 pthread_key_create</a></li>
    <li><a href="#pthreadsetspecific">2.8.26 pthread_setspecific</a></li>
    <li><a href="#pthreadgetspecific">2.8.27 pthread_getspecific</a></li>
    <li><a href="#pthreadkeydelete">2.8.28 pthread_key_delete</a></li>
  </ul>
  <p>
    <b>pthread Mutexes</b>.
  </p>
  <ul>
    <li><a href="#pthreadmutexattrinit">2.8.29 pthread_mutexattr_init</a></li>
    <li><a href="#pthreadmutexattrdestroy">2.8.30 pthread_mutexattr_destroy</a></li>
    <li><a href="#pthreadmutexattrgetpshared">2.8.31 pthread_mutexattr_getpshared</a></li>
    <li><a href="#pthreadmutexattrsetpshared">2.8.32 pthread_mutexattr_setpshared</a></li>
    <li><a href="#pthreadmutexattrgettype">2.8.33 pthread_mutexattr_gettype</a></li>
    <li><a href="#pthreadmutexattrsettype">2.8.34 pthread_mutexattr_settype</a></li>
    <li><a href="#pthreadmutexattrgetprotocol">2.8.35 pthread_mutexattr_getprotocol</a></li>
    <li><a href="#pthreadmutexattrsetprotocol">2.8.36 pthread_mutexattr_setprotocol</a></li>
    <li><a href="#pthreadmutexinit">2.8.37 pthread_mutex_init</a></li>
    <li><a href="#pthreadmutexdestrory">2.8.38 pthread_mutex_destroy</a></li>
    <li><a href="#pthreadmutexlock">2.8.39 pthread_mutex_lock</a></li>
    <li><a href="#pthreadmutextimedlock">2.8.40 pthread_mutex_timedlock</a></li>
    <li><a href="#pthreadmutextrylock">2.8.41 pthread_mutex_trylock</a></li>
    <li><a href="#pthreadmutexunlock">2.8.42 pthread_mutex_unlock</a></li>
  </ul>
  <p>
    <b>Condition Variables</b>.
  </p>
  <ul>
    <li><a href="#pthreadconaddrinit">2.8.42 pthread_condattr_init</a></li>
    <li><a href="#pthreadocndattrdestroy">2.8.43 pthread_condattr_destroy</a></li>
    <li><a href="#pthreadcondinit">2.8.44 pthread_cond_init</a></li>
    <li><a href="#pthreadconddestroy">2.8.45 pthread_cond_destroy</a></li>
    <li><a href="#pthreadcondbroadcast">2.8.46 pthread_cond_broadcast</a></li>
    <li><a href="#pthreadcondsignal">2.8.47 pthread_cond_signal</a></li>
    <li><a href="#pthreadcondwait">2.8.48 pthread_cond_wait</a></li>
    <li><a href="#pthreadcondtimedwait">2.8.49 pthread_cond_timedwait</a></li>
  </ul>
  <p>
    <b>Barriers</b>.
  </p>
  <ul>
    <li><a href="#pthreadbarrierattrinit">2.8.50 pthread_barrierattr_init</a></li>
    <li><a href="#pthreadbarrierattrdestroy">2.8.51 pthread_barrierattr_destroy</a></li>
    <li><a href="#pthreadbarrierattrsetpshared">2.8.52 pthread_barrierattr_setpshared</a></li>
    <li><a href="#pthreadbarrierattrgetpshared">2.8.53 pthread_barrierattr_getpshared</a></li>
    <li><a href="#pthreadbarrierinit">2.8.54 pthread_barrier_init</a></li>
    <li><a href="#pthreadbarrierdestroy">2.8.55 pthread_barrier_destroy</a></li>
    <li><a href="#pthreadbarrierwait">2.8.56 pthread_barrier_wait</a></li>
  </ul>
  <p>
    <b>Initialization</b>.
  </p>
  <ul>
    <li><a href="#pthreadonce">2.8.57 pthread_once</a></li>
  </ul>
  <p>
    <b>Signals</b>.
  </p>
  <ul>
    <li><a href="#pthreadkill">2.8.58 pthread_kill</a></li>
    <li><a href="#pthreadsigmask">2.8.59 pthread_sigmask</a></li>
  </ul>
</ul>
<p>
  No support for the following pthread interfaces is provided by NuttX:
</p>
<ul>
  <li><code>pthread_atfork</code>. register fork handlers.</li>
  <li><code>pthread_attr_getdetachstate</code>. get and set the detachstate attribute.</li>
  <li><code>pthread_attr_getguardsize</code>. get and set the thread guardsize attribute.</li>
  <li><code>pthread_attr_getinheritsched</code>. get and set the inheritsched attribute.</li>
  <li><code>pthread_attr_getscope</code>. get and set the contentionscope attribute.</li>
  <li><code>pthread_attr_getstack</code>. get and set stack attributes.</li>
  <li><code>pthread_attr_getstackaddr</code>. get and set the stackaddr attribute.</li>
  <li><code>pthread_attr_setdetachstate</code>. get and set the detachstate attribute.</li>
  <li><code>pthread_attr_setguardsize</code>. get and set the thread guardsize attribute.</li>
  <li><code>pthread_attr_setscope</code>. get and set the contentionscope attribute.</li>
  <li><code>pthread_attr_setstack</code>. get and set stack attributes.</li>
  <li><code>pthread_attr_setstackaddr</code>. get and set the stackaddr attribute.</li>
  <li><code>pthread_condattr_getclock</code>. set the clock selection condition variable attribute.</li>
  <li><code>pthread_condattr_getpshared</code>. get the process-shared condition variable attribute.</li>
  <li><code>pthread_condattr_setclock</code>. set the clock selection condition variable attribute.</li>
  <li><code>pthread_condattr_setpshared</code>. set the process-shared condition variable attribute.</li>
  <li><code>pthread_getconcurrency</code>. get and set the level of concurrency.</li>
  <li><code>pthread_getcpuclockid</code>. access a thread CPU-time clock.</li>
  <li><code>pthread_mutex_getprioceiling</code>. get and set the priority ceiling of a mutex.</li>
  <li><code>pthread_mutex_setprioceiling</code>. get and set the priority ceiling of a mutex.</li>
  <li><code>pthread_mutex_timedlock</code>. lock a mutex.</li>
  <li><code>pthread_mutexattr_getprioceiling</code>. get and set the prioceiling attribute of the mutex attributes object.</li>
  <li><code>pthread_mutexattr_setprioceiling</code>. get and set the prioceiling attribute of the mutex attributes object.</li>
  <li><code>pthread_rwlockattr_destroy</code>. destroy and initialize the read-write lock attributes object.</li>
  <li><code>pthread_rwlockattr_getpshared</code>. get and set the process-shared attribute of the read-write lock attributes object.</li>
  <li><code>pthread_rwlockattr_init</code>. destroy and initialize the read-write lock attributes object.</li>
  <li><code>pthread_rwlockattr_setpshared</code>. get and set the process-shared attribute of the read-write lock attributes object.</li>
  <li><code>pthread_setconcurrency</code>. get and set the level of concurrency.</li>
  <li><code>pthread_spin_destroy</code>. destroy or initialize a spin lock object.</li>
  <li><code>pthread_spin_init</code>. destroy or initialize a spin lock object.</li>
  <li><code>pthread_spin_lock</code>. lock a spin lock object.</li>
  <li><code>pthread_spin_trylock</code>. lock a spin lock object.</li>
  <li><code>pthread_spin_unlock</code>. unlock a spin lock object.</li>
</ul>

<H3><a name="pthreadattrinit">2.8.1 pthread_attr_init</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_init(pthread_attr_t *attr);
</pre>
<p>
<b>Description:</b>
Initializes a thread attributes object (attr) with default values
for all of the individual attributes used by the implementation.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_init()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
<p>
<H3><a name="pthreadattrdestroy">2.8.2 pthread_attr_destroy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_destroy(pthread_attr_t *attr);
</pre>
<p>
<b>Description:</b>
An attributes object can be deleted when it is no longer needed.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_destroy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
<p>
<H3><a name="pthreadattrsetschedpolity">2.8.3 pthread_attr_setschedpolicy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_setschedpolicy(pthread_attr_t *attr, int policy);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_setschedpolicy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadattrgetschedpolicy">2.8.4 pthread_attr_getschedpolicy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_getschedpolicy(pthread_attr_t *attr, int *policy);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_getschedpolicy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadattrsetschedparam">2.8.5 pthread_attr_getschedpolicy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_setschedparam(pthread_attr_t *attr,
                                   const struct sched_param *param);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_getschedpolicy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadattrgetschedparam">2.8.6 pthread_attr_getschedparam</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_getschedparam(pthread_attr_t *attr,
                                   struct sched_param *param);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_getschedparam()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadattrsetinheritsched">2.8.7 pthread_attr_setinheritsched</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
   #include &lt;pthread.h&gt;
    int pthread_attr_setinheritsched(pthread_attr_t *attr,
                                     int inheritsched);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_setinheritsched()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
<p>
<H3><a name="pthreadattrgetinheritsched">2.8.8 pthread_attr_getinheritsched</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
   #include &lt;pthread.h&gt;
     int pthread_attr_getinheritsched(const pthread_attr_t *attr,
                                      int *inheritsched);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_getinheritsched()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadattrsetstacksize">2.8.9 pthread_attr_setstacksize</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_setstacksize(pthread_attr_t *attr, long stacksize);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_setstacksize()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadattrgetstacksize">2.8.10 pthread_attr_getstacksize</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_attr_getstacksize(pthread_attr_t *attr, long *stackaddr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_attr_getstacksize()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcreate">2.8.11 pthread_create</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_create(pthread_t *thread, pthread_attr_t *attr,
                       pthread_startroutine_t startRoutine,
                       pthread_addr_t arg);
</pre>
<p>
<b>Description:</b>
To create a thread object and runnable thread, a routine
must be specified as the new thread's start routine.  An
argument may be passed to this routine, as an untyped
address; an untyped address may also be returned as the
routine's value.  An attributes object may be used to
specify details about the kind of thread being created.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_create()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreaddetach">2.8.12 pthread_detach</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_detach(pthread_t thread);
</pre>
<p>
<b>Description:</b>
A thread object may be "detached" to specify that the
return value and completion status will not be requested.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_detach()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadexit">2.8.13 pthread_exit</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    void pthread_exit(pthread_addr_t pvValue);
</pre>
<p>
<b>Description:</b>
A thread may terminate it's own execution.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_exit()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcancel">2.8.14 pthread_cancel</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cancel(pthread_t thread);
</pre>
<p>
<b>Description:</b>

<p>The <code>pthread_cancel()</code> function will request that thread be canceled.
The target thread's cancelability state, enabled, or disabled, determines when the cancellation takes effect:  When the cancellation is acted on, thread will be terminated.
When cancelability is disabled, all cancellations are held pending in the target thread until the thread re-enables cancelability.</p>

<p>The target thread's cancelability state determines how the cancellation is acted on:
Either asychronrously or deferred.
Asynchronous cancellations we be acted upon immediately (when enabled), interrupting the thread with its processing in an abritray state.</p>

<p>When cancelability is deferred, all cancellations are held pending in the target thread until the thread changes the cancelability type or a <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:cancellation-points">Cancellation Point</a> function such as <a href="#pthreadtestcancel"><code>pthread_testcancel()</code></a> is entered.</p>

<p>
<b>Input Parameters:</b>
<p>
<ul>
<li><code>thread</code>.
Identifies the thread to be canceled.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_cancel()</code> function will return zero (<code>OK</code>).
Otherwise, an error number will be returned to indicate the error:
<p>
<ul>
<li><code>ESRCH</code>.
No thread could be found corresponding to that specified by the given thread ID.</li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.  Except:</p>
<ul>
<li>The thread-specific data destructor functions will not be called for the thread.
These destructors are not currently supported.</li>
</ul>

<H3><a name="pthreadsetcancelstate">2.8.15 pthread_setcancelstate</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_setcancelstate(int state, int *oldstate);
</pre>
<p>
<b>Description:</b>
</p>
<p>
The <code>pthread_setcancelstate()</code> function atomically
sets both the calling thread's cancelability state to the indicated
state and returns the previous cancelability  state at the location
referenced by oldstate.
Legal values for state are PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DISABLE.
</p>
<p>
Any pending thread cancellation may occur at the time that the
cancellation state is set to PTHREAD_CANCEL_ENABLE.
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
<li><code>state</code>
New cancellation state.  One of PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE.</li>
<li><code>oldstate</code>.
Location to return the previous cancellation state.
</ul>
</p>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_setcancelstate()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be returned to indicate the error:
<p>
<ul>
<li><code>ESRCH</code>.
No thread could be found corresponding to that specified by the given thread ID.</li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadsetcanceltype">2.8.16 pthread_setcanceltype</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_setcanceltype(int type, FAR int *oldtype);
</pre>
<p>
<b>Description:</b>
The <code>pthread_setcanceltype()</code> function atomically both sets the calling thread's cancelability type to the indicated type and returns the previous cancelability type at the location referenced by <code>oldtype</code>.
Legal values for type are <code>PTHREAD_CANCEL_DEFERRED</code> and <code>PTHREAD_CANCEL_ASYNCHRONOUS</code>.
</p>
<p>
The cancelability state and type of any newly created threads are <code>PTHREAD_CANCEL_ENABLE</code> and <code>PTHREAD_CANCEL_DEFERRED respectively</code>.
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
<li><code>type</code>
New cancellation state.  One of <code>PTHREAD_CANCEL_DEFERRED</code> or <code>PTHREAD_CANCEL_ASYNCHRONOUS</code>.</li>
<li><code>oldtype</code>.
Location to return the previous cancellation type.
</ul>
</p>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_setcancelstate()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error.
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadtestcancel">2.8.17 pthread_testcancel</a></H3>
<p>
<b>Function Prototype:</b>
</p>
<p>
<pre>
    #include &lt;pthread.h&gt;
    void pthread_testcancel(void);
</pre>
</p>
<p>
<b>Description:</b>
</p>
<p>
The <code>pthread_testcancel()</code> function creates a <a href="http://www.nuttx.org/doku.php?id=wiki:nxinternal:cancellation-points">Cancellation Point</a> in the calling thread.
The <code>pthread_testcancel()</code> function has no effect if cancelability is disabled.
</p>
<p>
<b>Input Parameters:</b> None
</p>
<p>
<b>Returned Value:</b> None
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadcleauppop">2.8.18 pthread_cleanup_pop</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    void pthread_cleanup_pop(int execute);
</pre>
<p>
<b>Description:</b>
</p>
<p>
The <code>pthread_cleanup_pop()</code> function will remove the routine at the top of the calling thread's cancellation cleanup stack and optionally invoke it (if <code>execute</code> is non-zero).
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>execute</code>.  Execute the popped cleanup function immediately.</li>
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
If successful, the <code>pthread_setcancelstate()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadcleauppush">2.8.19 pthread_cleanup_push</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    void pthread_cleanup_push(CODE void (*routine)(FAR void *), FAR void *arg);
</pre>
<p>
<b>Description:</b>
</p>
<p>
The <code>pthread_cleanup_push()</code> function will push the specified cancellation cleanup handler routine onto the calling thread's cancellation cleanup stack.
</p>
<p>
The cancellation cleanup handler will be popped from the cancellation cleanup stack and invoked with the argument arg when:
</p>
<p>
<ul>
<li>The thread exits (that is, calls <code>pthread_exit()</code>).</li>
<li>The thread acts upon a cancellation request.</li>
<li>The thread calls <code>pthread_cleanup_pop()</code> with a non-zero execute argument.</li>
</ul>
</p>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>routine</code>.  The cleanup routine to be pushed on the cleanup stack.</li>
  <li><code>arg</code>.  An argument that will accompany the callback.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_setcancelstate()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error.
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadjoin">2.8.20 pthread_join</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_join(pthread_t thread, pthread_addr_t *ppvValue);
</pre>
<p>
<b>Description:</b>
A thread can await termination of another thread and retrieve
the return value of the thread.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_join()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadyield">2.8.21 pthread_yield</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    void pthread_yield(void);
</pre>
<p>
<b>Description:</b>
A thread may tell the scheduler that its processor can be
made available.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li>None</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
<ul>
  <li>None. The <code>pthread_yield()</code> function always succeeds.</li>
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> This call is nonstandard, but present on several
other systems. Use the POSIX <a href="#sched_yield"><code>sched_yield()</code></a> instead.

<H3><a name="pthreadself">2.8.22 pthread_self</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    pthread_t pthread_self(void);
</pre>
<p>
<b>Description:</b>
A thread may obtain a copy of its own thread handle.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li>None</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_self()</code> function will return
copy of caller's thread handle.  Otherwise, in exceptional circumstances,
the negated error code <code>-ESRCH</code> can be returned if the system
cannot deduce the identity of the calling thread.
</p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name. The <code>-ESRCH</code> return value is
non-standard; POSIX says <code>pthread_self()</code> must always succeed.
NuttX implements <code>pthread_self()</code> as a macro only, not as a
function as required by POSIX.
</p>

<H3><a name="pthreadgetschedparam">2.8.23 pthread_getschedparam</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_getschedparam(pthread_t thread, int *policy,
                              struct sched_param *param);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_getschedparam()</code> functions will get the
  scheduling policy and parameters of threads.
  For <code>SCHED_FIFO</code> and <code>SCHED_RR</code>, the only
  required member of the <code>sched_param</code> structure is the
  priority <code>sched_priority</code>.
</p>
<p>
  The <code>pthread_getschedparam()</code> function will retrieve the
  scheduling policy and scheduling parameters for the thread whose thread
  ID is given by <code>thread</code> and will store those values in
  <code>policy</code> and <code>param</code>, respectively.
  The priority value returned from <code>pthread_getschedparam()</code>
  will be the value specified by the most recent <code>pthread_setschedparam()</code>,
  <code>pthread_setschedprio()</code>, or <code>pthread_create()</code> call
  affecting the target thread.
  It will not reflect any temporary adjustments to its priority (such as might
  result of any priority inheritance, for example).
</p>
<p>
  The policy parameter may have the value <code>SCHED_FIFO</code>, <code>SCHED_RR</code>, or <code>SCHED_SPORADIC</code>.
  <code>SCHED_RR</code> requires the configuration setting <code>CONFIG_RR_INTERVAL &gt; 0</code>;
  <code>SCHED_SPORADIC</code> requires the configuration setting <code>CONFIG_SCHED_SPORADIC=y</code>.
  (<code>SCHED_OTHER</code> and non-standard scheduler policies, in particular, are not supported).
  The <code>SCHED_FIFO</code> and <code>SCHED_RR</code> policies will have a single
  scheduling parameter:
</p>
<ul>
  <li><code>sched_priority</code>
    The thread priority.
  </ul>
</ul>
<p>
  The <code>SCHED_SPORADIC</code> policy has four additional scheduling parameters:
</p>
<ul>
  <li><code>sched_ss_low_priority</code>
    Low scheduling priority for sporadic server.
  </li>
  <li><code>sched_ss_repl_period</code>
    Replenishment period for sporadic server.
  </li>
  <li><code>sched_ss_init_budget</code>
    Initial budget for sporadic server.
  </li>
  <li><code>sched_ss_max_repl</code>
    Maximum pending replenishments for  sporadic server.
  </li>
</ul>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>thread</code>.
    The ID of thread whose scheduling parameters will be queried.
  </li>
  <li>
    <code>policy</code>.
    The location to store the thread's scheduling policy.
  </li>
  <li>
    <code>param</code>.
    The location to store the thread's priority.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  0 (<code>OK</code>) if successful.
  Otherwise, the error code <code>ESRCH</code> if the value specified by
  <code>thread</code> does not refer to an existing thread.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadsetschedparam">2.8.24 pthread_setschedparam</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_setschedparam(pthread_t thread, int policy,
                              const struct sched_param *param);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_setschedparam()</code> functions will set the scheduling policy
  and parameters of threads.
  For <code>SCHED_FIFO</code> and <code>SCHED_RR</code>, the only required member
  of the <code>sched_param</code> structure is the priority <code>sched_priority</code>.
</p>
</p>
  The <code>pthread_setschedparam()</code> function will set the scheduling policy
  and associated scheduling parameters for the thread whose thread ID is given by
  <code>thread</code> to the policy and associated parameters provided in
  <code>policy</code> and <code>param</code>, respectively.
</p>
<p>
  The policy parameter may have the value <code>SCHED_FIFO</code> or <code>SCHED_RR</code>.
  (<code>SCHED_OTHER</code> and <code>SCHED_SPORADIC</code>, in particular, are not supported).
  The <code>SCHED_FIFO</code> and <code>SCHED_RR</code> policies will have a single
  scheduling parameter, <code>sched_priority</code>.
</p>
<p>
  If the <code>pthread_setschedparam()</code> function fails, the scheduling
  parameters will not be changed for the target thread.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>thread</code>.
    The ID of thread whose scheduling parameters will be modified.
  </li>
  <li>
    <code>policy</code>.
    The new scheduling policy of the thread.
    Either <code>SCHED_FIFO</code> or <code>SCHED_RR</code>.
    <code>SCHED_OTHER</code> and <code>SCHED_SPORADIC</code> are not supported.
  </li>
  <li>
    <code>param</code>.
    The location to store the thread's priority.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<p>
  If successful, the <code>pthread_setschedparam()</code> function will return
  zero (<code>OK</code>).  Otherwise, an error number will be
  returned to indicate the error:
</p>
<ul>
  <li>
    <code>EINVAL</code>.
    The value specified by <code>policy</code> or one of the scheduling parameters
    associated with the scheduling policy <code>policy</code> is invalid.
  </li>
  <li>
    <code>ENOTSUP</code>.
    An attempt was made to set the policy or scheduling parameters to an unsupported
    value (<code>SCHED_OTHER</code> and <code>SCHED_SPORADIC</code> in particular are
    not supported)
  </li>
  <li>
    <code>EPERM</code>.
    The caller does not have the appropriate permission to set either the scheduling
    parameters or the scheduling policy of the specified thread.
    Or, the implementation does not allow the application to modify one of the
    parameters to the value specified.

  </li>
  <li>
    <code>ESRCH</code>.
    The value specified by thread does not refer to a existing thread.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadkeycreate">2.8.25 pthread_key_create</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_key_create(pthread_key_t *key, void (*destructor)(void*))
</pre>
<p>
<b>Description:</b>
</p>
<p>
This function creates a thread-specific data key visible
to all threads in the system.  Although the same key value
may be used by different threads, the values bound to
the key by <code>pthread_setspecific()</code> are maintained on a
per-thread basis and persist for the life of the calling
thread.
</p>
<p>
Upon key creation, the value <code>NULL</code> will be associated with
the new key in all active threads.  Upon thread
creation, the value <code>NULL</code> will be associated with all
defined keys in the new thread.
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
<li><code>key</code> is a pointer to the key to create.
<li><code>destructor</code> is an optional destructor function that may
be associated with each key that is invoked when a
thread exits.  However, this argument is ignored in
the current implementation.
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
If successful, the <code>pthread_key_create()</code> function will
store the newly created key value at <code>*key</code> and return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<ul>
<li><code>EAGAIN</code>.  The system lacked sufficient resources
to create another thread-specific data key, or the
system-imposed limit on the total number of keys
per task {<code>PTHREAD_KEYS_MAX</code>} has been exceeded.
<li><code>ENOMEM</code>  Insufficient memory exists to create the key.
</ul>
</p>
<p>
<b>Assumptions/Limitations:</b>
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
<ul>
<li>The present implementation ignores the <code>destructor</code> argument.
</ul>
</p>

<H3><a name="pthreadsetspecific">2.8.26 pthread_setspecific</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_setspecific(pthread_key_t key, void *value)
</pre>
<p>
<b>Description:</b>
<p>
The <code>pthread_setspecific()</code> function associates a thread-specific
value with a key obtained via a previous call to <code>pthread_key_create()</code>.
Different threads may bind different values to the same key.  These values are
typically pointers to blocks of dynamically allocated memory that have been
reserved for use by the calling thread.
<p>
The effect of calling <code>pthread_setspecific()</code> with a key value
not obtained from <code>pthread_key_create()</code> or after a key has been
deleted with <code>pthread_key_delete()</code> is undefined.
<p>
<b>Input Parameters:</b>
<p>
<ul>
<li><code>key</code>.  The data key to set the binding for.
<li><code>value</code>. The value to bind to the key.
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, <code>pthread_setspecific()</code> will return zero (<code>OK</code>).
Otherwise, an error number will be returned:
<p>
<ul>
<li><code>ENOMEM</code>.  Insufficient memory exists to associate the value
with the key.
<li><code>EINVAL</code>.  The key value is invalid.
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
<ul>
<li><code>pthread_setspecific()</code> may be called from a thread-specific data
destructor function.
</ul>

<H3><a name="pthreadgetspecific">2.8.27 pthread_getspecific</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    void *pthread_getspecific(pthread_key_t key)
</pre>
<p>
<b>Description:</b>
<p>
The <code>pthread_getspecific()</code> function returns the value
currently bound to the specified key on behalf of the
calling thread.
<p>
The effect of calling <code>pthread_getspecific()</code> with a key value
not obtained from <code>pthread_key_create()</code> or after a key has been
deleted with <code>pthread_key_delete()</code> is undefined.
<p>
<b>Input Parameters:</b>
<p>
<ul>
<li><code>key</code>.  The data key to get the binding for.
</ul>
<p>
<b>Returned Value:</b>
<p>
The function <code>pthread_getspecific()</code> returns the thread-specific
data associated with the given key.  If no thread specific data is
associated with the key, then the value <code>NULL</code> is returned.
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.
<ul>
<li><code>pthread_getspecific()</code> may be called from a thread-specific data
destructor function.
</ul>

<H3><a name="pthreadkeydelete">2.8.28 pthread_key_delete</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_key_delete(pthread_key_t key)
</pre>
<p>
<b>Description:</b>
<p>
This POSIX function deletes a thread-specific data key previously
returned by <code>pthread_key_create()</code>. No cleanup actions
are done for data structures related to the deleted key or associated
thread-specific data in any threads. It is undefined behavior to use
<code>key</code> after it has been deleted.
<p>
<b>Input Parameters:</b>
<p>
<ul>
<li><code>key</code>.  The key to delete
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_key_delete()</code> function will
return zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
<li><code>EINVAL</code>.  The parameter <code>key</code> is invalid.
</ul>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutexattrinit">2.8.29 pthread_mutexattr_init</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutexattr_init(pthread_mutexattr_t *attr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutexattr_init()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutexattrdestroy">2.8.30 pthread_mutexattr_destroy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutexattr_destroy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutexattrgetpshared">2.8.31 pthread_mutexattr_getpshared</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutexattr_getpshared(pthread_mutexattr_t *attr,
                                     int *pshared);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutexattr_getpshared()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutexattrsetpshared">2.8.32 pthread_mutexattr_setpshared</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
   int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr,
                                    int pshared);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutexattr_setpshared()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<h3><a name="pthreadmutexattrgettype">2.8.33 pthread_mutexattr_gettype</a></h3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
#ifdef CONFIG_PTHREAD_MUTEX_TYPES
    int pthread_mutexattr_gettype(const pthread_mutexattr_t *attr, int *type);
#endif
</pre>
<p>
<b>Description:</b> Return the mutex type from the mutex attributes.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>attr</code>. The mutex attributes to query</li>
  <li><code>type</code>.  Location to return the mutex type.  See
    <a href="#pthreadmutexattrsettype"><code>pthread_mutexattr_settype()</code></a>
    for a description of possible mutex types that may be returned.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutexattr_settype()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>EINVAL</code>. Parameters <code>attr</code> and/or <code>attr</code> are invalid.</li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.

<h3><a name="pthreadmutexattrsettype">2.8.34 pthread_mutexattr_settype</a></h3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
#ifdef CONFIG_PTHREAD_MUTEX_TYPES
    int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type);
#endif
</pre>
<p>
<b>Description:</b> Set the mutex type in the mutex attributes.
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>attr</code>. The mutex attributes in which to set the mutex type.</li>
  <li><code>type</code>. The mutex type value to set.  The following values are supported:
  <ul>
    <li><code>PTHREAD_MUTEX_NORMAL</code>.  This type of mutex does not detect deadlock. A thread
      attempting to re-lock this mutex without first unlocking it will deadlock.
      Attempting to unlock a mutex locked by a different thread results in undefined
      behavior. Attempting to unlock an unlocked mutex results in undefined behavior. </li>
    <li><code>PTHREAD_MUTEX_ERRORCHECK</code>.  This type of mutex provides error checking.
      A thread attempting to re-lock this mutex without first unlocking it will return with an error.
      A thread attempting to unlock a mutex which another thread has locked will return with an error.
      A thread attempting to unlock an unlocked mutex will return with an error.</li>
    <li><code>PTHREAD_MUTEX_RECURSIVE</code>.  A thread attempting to re-lock this mutex without first
      unlocking it will succeed in locking the mutex. The re-locking deadlock which can occur with mutexes
      of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex
      require the same number of unlocks to release the mutex before another thread can acquire the mutex.
      A thread attempting to unlock a mutex which another thread has locked will return with an error.
      A thread attempting to unlock an unlocked mutex will return with an error.</li>
    <li><code>PTHREAD_MUTEX_DEFAULT</code>.  The default mutex type (PTHREAD_MUTEX_NORMAL).</li>
  </ul>
  <p>
    In NuttX, <code>PTHREAD_MUTEX_NORMAL</code> is not implemented.  Rather, the behavior described
    for <code>PTHREAD_MUTEX_ERRORCHECK</code> is the <i>normal</i> behavior.
  </p>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutexattr_settype()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>EINVAL</code>. Parameters <code>attr</code> and/or <code>attr</code> are invalid.</li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.

<H3><a name="pthreadmutexattrgetprotocol">2.8.35 pthread_mutexattr_getprotocol</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutexattr_getprotocol(FAR const pthread_mutexattr_t *attr,
                                      FAR int *protocol);
</pre>
<p>
<b>Description:</b> Return the value of the mutex protocol attribute..
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>attr</code>. A pointer to the mutex attributes to be queried</li>
  <li><code>protocol</code>. The user provided location in which to store the protocol value.  May be one of <code>PTHREAD_PRIO_NONE</code>, or <code>PTHREAD_PRIO_INHERIT</code>, <code>PTHREAD_PRIO_PROTECT</code>.</li>
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
If successful, the <code>pthread_mutexattr_getprotocol()</code> function will return zero (<code>OK</code>).
Otherwise, an error number will be returned to indicate the error.
</p>
<p>
<b>Assumptions/Limitations:</b>
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadmutexattrsetprotocol">2.8.36 pthread_mutexattr_setprotocol</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutexattr_setprotocol(FAR pthread_mutexattr_t *attr,
                                      int protocol);
</pre>
<p>
<b>Description:</b> Set mutex protocol attribute.  See the paragraph <a href="#lockingvssignaling">Locking versus Signaling Semaphores</a> for some important information about the use of this interface.
</p>
<p>
<b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>attr</code>. A pointer to the mutex attributes to be modified</li>
  <li><code>protocol</code>. The new protocol to use.  One of <code>PTHREAD_PRIO_NONE</code>, or <code>PTHREAD_PRIO_INHERIT</code>, <code>PTHREAD_PRIO_PROTECT</code>.  <code>PTHREAD_PRIO_INHERIT</code> is supported only if  <code>CONFIG_PRIORITY_INHERITANCE</code> is defined;  <code>PTHREAD_PRIO_PROTECT</code> is not currently supported in any configuration.</li>
</ul>
</p>
<p>
<b>Returned Value:</b>
</p>
<p>
If successful, the <code>pthread_mutexattr_setprotocol()</code> function will return zero (<code>OK</code>).
Otherwise, an error number will be returned to indicate the error.
</p>
<p>
<b>Assumptions/Limitations:</b>
</p>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<H3><a name="pthreadmutexinit">2.8.37 pthread_mutex_init</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutex_init(pthread_mutex_t *mutex,
                           pthread_mutexattr_t *attr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutex_init()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.

<H3><a name="pthreadmutexdestrory">2.8.38 pthread_mutex_destroy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutex_destroy(pthread_mutex_t *mutex);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutex_destroy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutexlock">2.8.39 pthread_mutex_lock</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutex_lock(pthread_mutex_t *mutex);
</pre>
<p>
<b>Description:</b>
   The mutex object referenced by mutex is locked by calling <code>pthread_mutex_lock()</code>.
   If the mutex is already locked, the calling thread blocks until the mutex
   becomes available. This operation returns with the mutex object referenced
   by mutex in the locked state with the calling thread as its owner.
</p>
<p>
   If the mutex type is <code>PTHREAD_MUTEX_NORMAL</code>, deadlock detection is not provided.
   Attempting to re-lock the mutex causes deadlock. If a thread attempts to unlock
   a mutex that it has not locked or a mutex which is unlocked, undefined behavior
   results.
</p>
<p>
   In NuttX, <code>PTHREAD_MUTEX_NORMAL</code> is not implemented.  Rather, the behavior described
   for <code>PTHREAD_MUTEX_ERRORCHECK</code> is the <i>normal</i> behavior.
</p>
<p>
   If the mutex type is <code>PTHREAD_MUTEX_ERRORCHECK</code>, then error checking is provided.
   If a thread attempts to re-lock a mutex that it has already locked, an error
   will be returned. If a thread attempts to unlock a mutex that it has not
   locked or a mutex which is unlocked, an error will be returned.
</p>
<p>
   If the mutex type is <code>PTHREAD_MUTEX_RECURSIVE</code>, then the mutex maintains the concept
   of a lock count. When a thread successfully acquires a mutex for the first time,
   the lock count is set to one. Every time a thread re-locks this mutex, the lock count
   is incremented by one. Each time the thread unlocks the mutex, the lock count is
   decremented by one. When the lock count reaches zero, the mutex becomes available
   for other threads to acquire. If a thread attempts to unlock a mutex that it has
   not locked or a mutex which is unlocked, an error will be returned.
</p>
<p>
   If a signal is delivered to a thread waiting for a mutex, upon return from
   the signal handler the thread resumes waiting for the mutex as if it was
   not interrupted.
</p>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>mutex</code>.  A reference to the mutex to be locked.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutex_lock()</code> function will return zero (<code>OK</code>).
Otherwise, an error number will be returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<p>Note that this function will never return the error EINTR.</p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutextimedlock">2.8.40 pthread_mutex_timedlock</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutex_timedlock(pthread_mutex_t *mutex, const struct timespec *abs_timeout);
</pre>
<p>
<b>Description:</b>
  The <code>pthread_mutex_timedlock()</code> function will lock the mutex object referenced by <code>mutex</code>.
  If the mutex is already locked, the calling thread will block until the mutex becomes available
  as in the <a href="#pthreadmutexlock"><code>pthread_mutex_lock()</code></a> function.
  If the mutex cannot be locked without waiting for another thread to unlock the mutex, this wait
  will be terminated when the specified <code>abs_timeout</code> expires.
</p>
<p>
  The timeout will expire when the absolute time specified by <code>abs_timeout</code> passes,
  as measured by the clock on which timeouts are based (that is, when the value of that clock
  equals or exceeds <code>abs_timeout</code>), or if the absolute time specified by
  <code>abs_timeout</code> has already been passed at the time of the call.
</p>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>mutex</code>.  A reference to the mutex to be locked.</li>
  <li><code>abs_timeout</code>.  Maximum wait time (with <code>NULL</code> meaning to wait forever).</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutex_trylock()</code> function will return zero (<code>OK</code>).
Otherwise, an error number will be returned to indicate the error.
Note that the errno <code>EINTR</code> is never returned by <code>pthread_mutex_timedlock()</code>.
The returned errno is ETIMEDOUT if the mutex could not be locked before the specified timeout expired
</p>
<p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b>
  Comparable to the POSIX interface of the same name.
  This implementation does not return <code>EAGAIN</code> when the mutex could not be acquired because the maximum number of recursive locks for mutex has been exceeded.
</p>

<H3><a name="pthreadmutextrylock">2.8.41 pthread_mutex_trylock</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutex_trylock(pthread_mutex_t *mutex);
</pre>
<p>
<b>Description:</b>
   The function <code>pthread_mutex_trylock()</code> is identical to <a href="#pthreadmutexlock"><code>pthread_mutex_lock()</code></a>
   except that if the mutex object referenced by mutex is currently locked
   (by any thread, including the current thread), the call returns immediately
   with the <code>errno</code> <code>EBUSY</code>.
<p>
   If a signal is delivered to a thread waiting for a mutex, upon return from
   the signal handler the thread resumes waiting for the mutex as if it was
   not interrupted.
</p>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>mutex</code>.  A reference to the mutex to be locked.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutex_trylock()</code> function will return zero (<code>OK</code>).
Otherwise, an error number will be returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<p>Note that this function will never return the error EINTR.</p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadmutexunlock">2.8.42 pthread_mutex_unlock</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_mutex_unlock(pthread_mutex_t *mutex);
</pre>
<p>
<b>Description:</b>
<p>
   The <code>pthread_mutex_unlock()</code> function releases the mutex object referenced
   by mutex. The manner in which a mutex is released is dependent upon the
   mutex's type attribute. If there are threads blocked on the mutex object
   referenced by mutex when <code>pthread_mutex_unlock()</code> is called, resulting in
   the mutex becoming available, the scheduling policy is used to determine
   which thread will acquire the mutex. (In the case of <code>PTHREAD_MUTEX_RECURSIVE</code>
   mutexes, the mutex becomes available when the count reaches zero and the
   calling thread no longer has any locks on this mutex).
</p>
<p>
   If a signal is delivered to a thread waiting for a mutex, upon return from
   the signal handler the thread resumes waiting for the mutex as if it was
   not interrupted.
</p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>mutex</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_mutex_unlock()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<p>Note that this function will never return the error EINTR.</p>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadconaddrinit">2.8.42 pthread_condattr_init</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_condattr_init(pthread_condattr_t *attr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_condattr_init()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadocndattrdestroy">2.8.43 pthread_condattr_destroy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_condattr_destroy(pthread_condattr_t *attr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_condattr_destroy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcondinit">2.8.44 pthread_cond_init</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *attr);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_cond_init()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadconddestroy">2.8.45 pthread_cond_destroy</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cond_destroy(pthread_cond_t *cond);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_cond_destroy()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcondbroadcast">2.8.46 pthread_cond_broadcast</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cond_broadcast(pthread_cond_t *cond);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_cond_broadcast()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcondsignal">2.8.47 pthread_cond_signal</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cond_signal(pthread_cond_t *dond);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
<b>Returned Value:</b>
<p>
If successful, the <code>pthread_cond_signal()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcondwait">2.8.48 pthread_cond_wait</a></H3>
<p>
<b>Function Prototype:</b>
<p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
</pre>
<p>
<b>Description:</b>
<p>
<b>Input Parameters:</b>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
  <b>Returned Value:</b>
<p>
If successful, the <code>pthread_cond_wait()</code> function will return
zero (<code>OK</code>).  Otherwise, an error number will be
returned to indicate the error:
<p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<b>Assumptions/Limitations:</b>
<p>
<b>POSIX Compatibility:</b> Comparable to the POSIX
interface of the same name.

<H3><a name="pthreadcondtimedwait">2.8.49 pthread_cond_timedwait</a></H3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex,
                               const struct timespec *abstime);
</pre>
<p>
  <b>Description:</b>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>To be provided</code>.</li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<p>
  If successful, the <code>pthread_cond_timedwait()</code> function will return
  zero (<code>OK</code>).  Otherwise, an error number will be
  returned to indicate the error:
</p>
<ul>
  <li><code>To be provided</code>. </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierattrinit">2.8.50 pthread_barrierattr_init</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrierattr_init(FAR pthread_barrierattr_t *attr);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_barrierattr_init()</code> function will initialize a barrier
  attribute object <code>attr</code> with the default value for all of the attributes
  defined by the implementation.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>attr</code>. Barrier attributes to be initialized.
</li>
</ul>
<p>
  <b>Returned Value:</b>
  0 (<code>OK</code>) on success or <code>EINVAL</code> if <code>attr</code> is invalid.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierattrdestroy">2.8.51 pthread_barrierattr_destroy</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrierattr_destroy(FAR pthread_barrierattr_t *attr);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_barrierattr_destroy()</code> function will destroy a barrier attributes object.
  A destroyed attributes object can be reinitialized using <code>pthread_barrierattr_init()</code>;
  the results of otherwise referencing the object after it has been destroyed are undefined.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>attr</code>. Barrier attributes to be destroyed.
</li>
</ul>
<p>
  <b>Returned Value:</b> 0 (<code>OK</code>) on success or <code>EINVAL</code> if attr is invalid.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierattrsetpshared">2.8.52 pthread_barrierattr_setpshared</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrierattr_setpshared(FAR pthread_barrierattr_t *attr, int pshared);
</pre>
<p>
  <b>Description:</b>
  The process-shared attribute is set to <code>PTHREAD_PROCESS_SHARED</code> to permit
  a barrier to be operated upon by any thread that has access to the memory where the
  barrier is allocated.
  If the process-shared attribute is <code>PTHREAD_PROCESS_PRIVATE</code>, the barrier can
  only be operated upon by threads created within the same process as the thread that
  initialized the barrier.
  If threads of different processes attempt to operate on such a barrier, the behavior is undefined.
  The default value of the attribute is <code>PTHREAD_PROCESS_PRIVATE</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>attr</code>. Barrier attributes to be modified.</li>
  <li><code>pshared</code>. The new value of the pshared attribute.</li>
</ul>
<p>
  <b>Returned Value:</b>  0 (<code>OK</code>) on success or <code>EINVAL</code> if either
  <code>attr</code> is invalid or <code>pshared</code> is not one of
  <code>PTHREAD_PROCESS_SHARED</code> or <code>PTHREAD_PROCESS_PRIVATE</code>.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierattrgetpshared">2.8.53 pthread_barrierattr_getpshared</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrierattr_getpshared(FAR const pthread_barrierattr_t *attr, FAR int *pshared);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_barrierattr_getpshared()</code> function will obtain the value of the
  process-shared attribute from the attributes object referenced by <code>attr</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>attr</code>. Barrier attributes to be queried.</li>
  <li><code>pshared</code>. The location to stored the current value of the pshared attribute.</li>
</ul>
<p>
  <b>Returned Value:</b> 0 (<code>OK</code>) on success or <code>EINVAL</code> if
  either <code>attr</code> or <code>pshared</code> is invalid.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierinit">2.8.54 pthread_barrier_init</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrier_init(FAR pthread_barrier_t *barrier,
                             FAR const pthread_barrierattr_t *attr, unsigned int count);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_barrier_init()</code> function allocates any resources required to
  use the barrier referenced by <code>barrier</code> and initialized the barrier with
  the attributes referenced by <code>attr</code>.
  If <code>attr</code> is NULL, the default barrier attributes will be used.
  The results are undefined if <code>pthread_barrier_init()</code> is called when any
  thread is blocked on the barrier.
  The results are undefined if a barrier is used without first being initialized.
  The results are undefined if <code>pthread_barrier_init()</code> is called specifying
  an already initialized barrier.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>barrier</code>.
    The barrier to be initialized.
  </li>
  <li>
    <code>attr</code>.
    Barrier attributes to be used in the initialization.
  </li>
  <li>
    <code>count</code>.
    The count to be associated with the barrier.
    The count argument specifies the number of threads that must call
    <code>pthread_barrier_wait()</code> before any of them successfully return from the call.
    The value specified by count must be greater than zero.
  </li>
</ul>
<p>
  <b>Returned Value:</b> 0 (<code>OK</code>) on success or one of the following error numbers:
</p>
<ul>
  <li>
    <code>EAGAIN</code>.
    The system lacks the necessary resources to initialize another barrier.
  </li>
  <li>
    <code>EINVAL</code>.
    The <code>barrier</code> reference is invalid, or the values specified by <code>attr</code>
    are invalid, or the value specified by <code>count</code> is equal to zero.
  </li>
  <li>
    <code>ENOMEM</code>.
    Insufficient memory exists to initialize the barrier.
  </li>
  <li>
    <code>EBUSY</code>.
    The implementation has detected an attempt to reinitialize a barrier while it is in use.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierdestroy">2.8.55 pthread_barrier_destroy</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrier_destroy(FAR pthread_barrier_t *barrier);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_barrier_destroy()</code> function destroys the barrier referenced
  by <code>barrie</code> and releases any resources used by the barrier.
  The effect of subsequent use of the barrier is undefined until the barrier is
  reinitialized by another call to <code>pthread_barrier_init()</code>.
  The results are undefined if <code>pthread_barrier_destroy()</code> is called when
  any thread is blocked on the barrier, or if this function is called with an
  uninitialized barrier.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>barrier</code>. The barrier to be destroyed.</li>
</ul>
<p>
  <b>Returned Value:</b> 0 (<code>OK</code>) on success or one of the following error numbers:
</p>
<ul>
  <li>
    <code>EBUSY</code>.
    The implementation has detected an attempt to destroy a barrier while it is in use.
  </li>
  <li>
    <code>EINVAL</code>.
    The value specified by <code>barrier</code> is invalid.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadbarrierwait">2.8.56 pthread_barrier_wait</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_barrier_wait(FAR pthread_barrier_t *barrier);
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_barrier_wait()</code> function synchronizes participating
  threads at the barrier referenced by <code>barrier</code>.
  The calling thread is blocked until the required number of threads have called
  <code>pthread_barrier_wait()</code> specifying the same <code>barrier</code>.
  When the required number of threads have called <code>pthread_barrier_wait()</code>
  specifying the <code>barrier</code>, the constant <code>PTHREAD_BARRIER_SERIAL_THREAD</code>
  will be returned to one unspecified thread and zero will be returned to each of
  the remaining threads.
  At this point, the barrier will be reset to the state it had as a result of the most
  recent <code>pthread_barrier_init()</code> function that referenced it.
</p>
<p>
  The constant <code>PTHREAD_BARRIER_SERIAL_THREAD</code> is defined in
 <code>pthread.h</code> and its value must be distinct from any other value
  returned by <code>pthread_barrier_wait()</code>.
</p>
<p>
  The results are undefined if this function is called with an uninitialized barrier.
</p>
<p>
  If a signal is delivered to a thread blocked on a barrier, upon return from the
  signal handler the thread will resume waiting at the barrier if the barrier wait
  has not completed.
  Otherwise, the thread will continue as normal from the completed barrier wait.
  Until the thread in the signal handler returns from it, it is unspecified whether
  other threads may proceed past the barrier once they have all reached it.
</p>
<p>
  A thread that has blocked on a barrier will not prevent any unblocked thread that
  is eligible to use the same processing resources from eventually making forward
  progress in its execution.
  Eligibility for processing resources will be determined by the scheduling policy.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>barrier</code>. The barrier on which to wait.</li>
</ul>
<p>
  <b>Returned Value:</b> 0 (<code>OK</code>) on success or <code>EINVAL</code> if the barrier is not valid.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>


<h3><a name="pthreadonce">2.8.57 pthread_once</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;pthread.h&gt;
    int pthread_once(FAR pthread_once_t *once_control, CODE void (*init_routine)(void));
</pre>
<p>
  <b>Description:</b>
  The  first  call to <code>pthread_once()</code> by any thread with a given
  <code>once_control</code>, will call the <code>init_routine()</code> with no arguments.
  Subsequent calls to <code>pthread_once()</code> with the same <code>once_control</code> will have no effect.
  On return from <code>pthread_once()</code>, <code>init_routine()</code> will have completed.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>once_control</code>.
    Determines if <code>init_routine()</code> should be called.
    <code>once_control</code> should be declared and initialized as follows:
    <ul><pre>pthread_once_t once_control = PTHREAD_ONCE_INIT;
    </pre></ul>
    <code>PTHREAD_ONCE_INIT</code> is defined in <code>pthread.h</code>.
  </li>
  <li>
    <code>init_routine</code>.
    The initialization routine that will be called once.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  0 (<code>OK</code>) on success or <code>EINVAL</code> if either once_control or init_routine are invalid.
</p>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadkill">2.8.58 pthread_kill</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;signal.h&gt;
    #include &lt;pthread.h&gt;
    int pthread_kill(pthread_t thread, int signo)
</pre>
<p>
  <b>Description:</b>
  The <code>pthread_kill()</code> system call can be used to send any
  signal to a thread.  See <code>kill()</code> for further information
  as this is just a simple wrapper around the <code>kill()</code>
  function.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>thread</code>.
    The id of the thread to receive the signal. Only positive, non-zero values of <code>tthread</code>t are supported.
  </li>
  <li>
    <code>signo</code>.
    The signal number to send.  If <code>signo</code> is zero, no signal is sent, but all error checking is performed.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<p>
  On success, the signal was sent and zero is returned.
  On error one of the following error numbers is returned.
</p>
<ul>
  <li>
    <code>EINVAL</code>.
    An invalid signal was specified.
  </li>
  <li>
    <code>EPERM</code>.
    The thread does not have permission to send the signal to the target thread.
  </li>
  <li>
    <code>ESRCH</code>.
    No thread could be found corresponding to that specified by the given thread ID.
  </li>
  <li>
    <code>ENOSYS</code>.
    Do not support sending signals to process groups.
  </li>
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<h3><a name="pthreadsigmask">2.8.59 pthread_sigmask</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
    #include &lt;signal.h&gt;
    #include &lt;pthread.h&gt;
    int pthread_sigmask(int how, FAR const sigset_t *set, FAR sigset_t *oset);
</pre>
<p>
  <b>Description:</b>
  This function is a simple wrapper around <code>sigprocmask()</code>.
  See the <code>sigprocmask()</code> function description for further information.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>how</code>. How the signal mast will be changed:
    <ul>
      <li>
        <code>SIG_BLOCK</code>:
        The resulting set is the union of the current set and the signal set pointed to by <code>set</code>.
      </li>
      <li>
        <code>SIG_UNBLOCK</code>:
        The resulting set is the intersection of the current set and the complement of the signal set pointed to by <code>set</code>.
      </li>
      <li>
        <code>SIG_SETMASK</code>:
        The resulting set is the signal set pointed to by <code>set</code>.
      </li>
    </ul>
  </li>
  <li>
    <code>set</code>. Location of the new signal mask.
  </li>
  <li>
    <code>oset</code>. Location to store the old signal mask.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<ul>
  0 (<code>OK</code>) on success or <code>EINVAL</code> if <code>how</code> is invalid.
</ul>
<p>
  <b>Assumptions/Limitations:</b>
</p>
<p>
  <b>POSIX Compatibility:</b> Comparable to the POSIX interface of the same name.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Environ"><h2>2.9 Environment Variables</h2></a>
  </td>
  </tr>
</table>

<p><b>Overview</b>.
  NuttX supports environment variables that can be used to control the behavior of programs.
  In the spirit of NuttX the environment variable behavior attempts to emulate the behavior of
  environment variables in the multi-processing OS:
</p>
<ul>
  <li><b>Task environments</b>.
    When a new task is created using <a href="#taskcreate">task_create</a>, the environment
    of the child task is an inherited, exact copy of the environment of the parent.
    However, after child task has been created, subsequent operations by the child task on
    its environment does not alter the environment of the parent.
    No do operations by the parent effect the child's environment.
    The environments start identical but are independent and may diverge.
  </li>
  <li><b>Thread environments</b>.
    When a pthread is created using <a href="#pthreadcreate">pthread_create</a>, the child
    thread also inherits that environment of the parent.
    However, the child does not receive a copy of the environment but, rather, shares the same
    environment.
    Changes to the environment are visible to all threads with the same parentage.
  </li>
</ul>
<p><b>Programming Interfaces</b>.
  The following environment variable programming interfaces are provided by Nuttx and are
  described in detail in the following paragraphs.
</p>
<ul>
  <li><a href="#getenv">2.9.1 <code>getenv</code></a></li>
  <li><a href="#putenv">2.9.2 <code>putenv</code></a></li>
  <li><a href="#clearenv">2.9.3 <code>clearenv</code></a></li>
  <li><a href="#setenv">2.9.4 <code>setenv</code></a></li>
  <li><a href="#unsetenv">2.9.5 <code>unsetenv</code></a></li>
</ul>
<p><b>Disabling Environment Variable Support</b>.
  All support for environment variables can be disabled by setting <code>CONFIG_DISABLE_ENVIRON</code>
  in the board configuration file.
</p>

<h3><a name="getenv">2.9.1 <code>getenv</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
  #include &lt;stdlib.h&gt;
  FAR char *getenv(const char *name);
</pre>
<p>
  <b>Description:</b>
    The <code>getenv()</code> function searches the environment list for a string that
    matches the string pointed to by <code>name</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>name</code>.
    The name of the variable to find.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
   The value of the variable (read-only) or NULL on failure.
</p>

<h3><a name="putenv">2.9.2 <code>putenv</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
  #include &lt;stdlib.h&gt;
  int putenv(char *string);
</pre>
<p>
  <b>Description:</b>
  The <code>putenv()</code> function adds or changes the value of environment variables.
  The argument string is of the form <i>name=value</i>. If name does not already
  exist in  the  environment, then string is added to the environment. If
  name does exist, then the value of name in the environment is changed to
  value.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>string</code>
    <i>name=value</i> string describing the environment setting to add/modify.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  Zero on success.
</p>

<h3><a name="clearenv">2.9.3 <code>clearenv</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
  #include &lt;stdlib.h&gt;
  int clearenv(void);
</pre>
<p>
  <b>Description:</b>
  The <code>clearenv()</code> function clears the environment of all name-value pairs
  and sets the value of the external variable environ to NULL.
</p>
<p>
  <b>Input Parameters:</b>
  None
</p>
<p>
  <b>Returned Value:</b>
  Zero on success.
</p>

<h3><a name="setenv">2.9.4 <code>setenv</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
  #include &lt;stdlib.h&gt;
  int setenv(const char *name, const char *value, int overwrite);
</pre>
<p>
  <b>Description:</b>
  The <code>setenv()</code> function adds the variable <code>name</code> to the environment with the
  specified <code>value</code> if the variable <code>name</code> does not exist. If the <code>name</code>
  does exist in the environment, then its value is changed to <code>value</code> if <code>overwrite</code>
  is non-zero; if <code>overwrite</code> is zero, then the value of <code>name</code> is unaltered.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>name</code>
    The name of the variable to change.
  </li>
  <li>
    <code>value</code>
    The new value of the variable.
  </li>
  <li>
    <code>value</code>
    Replace any existing value if non-zero.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  Zero on success.
</p>

<h3><a name="unsetenv">2.9.5 <code>unsetenv</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
  #include &lt;stdlib.h&gt;
  int unsetenv(const char *name);
</pre>
<p>
  <b>Description:</b>
  The <code>unsetenv()</code> function deletes the variable <code>name</code> from the environment.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li>
    <code>name</code>
    The name of the variable to delete.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  Zero on success.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="FileSystem"><h2>2.10 File System Interfaces</h2></a>
  </td>
  </tr>
</table>

<ul>
  <li><a href="#FileSystemOverview">2.10.1 NuttX File System Overview</a></li>
  <li><a href="#driveroperations">2.10.2 Driver Operations</a></li>
  <li><a href="#directoryoperations">2.10.3 Directory Operations</a></li>
  <li><a href="#dirunistdops">2.10.4 UNIX Standard Operations</a></li>
  <li><a href="#standardio">2.10.5 Standard I/O</a></li>
  <li><a href="#standardlib">2.10.6 Standard Library</a></li>
  <li><a href="#aio">2.10.7 Asynchronous I/O</a></li>
  <li><a href="#stdstrings">2.10.8 Standard String Operations</a></li>
  <li><a href="#PipesNFifos">2.10.9 Pipes and FIFOs</a></li>
  <li><a href="#mmapxip">2.10.10 <code>mmap()</code> and eXecute In Place (XIP)</a></li>
</ul>

<h3><a name="FileSystemOverview">2.10.1 NuttX File System Overview</a></h3>

<p><b>Overview</b>.
  NuttX includes an optional, scalable file system.
  This file-system may be omitted altogether; NuttX does not depend on the presence
  of any file system.
</p>

<p><b>Pseudo Root File System</b>.
  A simple <i>in-memory</i>, <i>pseudo</i> file system can be enabled by default.
  This is an <i>in-memory</i> file system because it does not require any
  storage medium or block driver support.
  Rather, file system contents are generated on-the-fly as referenced via
  standard file system operations (open, close, read, write, etc.).
  In this sense, the file system is <i>pseudo</i> file system (in the
  same sense that the Linux <code>/proc</code> file system is also
  referred to as a pseudo file system).
</p>

<p>
  Any user supplied data or logic can be accessed via the pseudo-file system.
  Built in support is provided for character and block
  <a href="NuttxPortingGuide.html#DeviceDrivers">driver</a> <i>nodes</i> in the any
  pseudo file system directory.
  (By convention, however, all driver nodes should be in the <code>/dev</code>
  pseudo file system directory).
</p>

<p><b>Mounted File Systems</b>
  The simple in-memory file system can be extended my mounting block
  devices that provide access to true file systems backed up via some
  mass storage device.
  NuttX supports the standard <code>mount()</code> command that allows
  a block driver to be bound to a mount-point within the pseudo file system
  and to a a file system.
  At present, NuttX supports only the VFAT file system.
</p>

<p><b>Comparison to Linux</b>
  From a programming perspective, the NuttX file system appears very similar
  to a Linux file system.
  However, there is a fundamental difference:
  The NuttX root file system is a pseudo file system and true file systems may be
  mounted in the pseudo file system.
  In the typical Linux installation by comparison, the Linux root file system
  is a true file system and pseudo file systems may be mounted in the true,
  root file system.
  The approach selected by NuttX is intended to support greater scalability
  from the very tiny platform to the moderate platform.
</p>

<p><b>File System Interfaces</b>.
  The NuttX file system simply supports a set of standard, file system APIs
  (<code>open()</code>, <code>close()</code>, <code>read()</code>, <code>write</code>, etc.)
  and a registration mechanism that allows devices drivers to a associated with <i>nodes</i>
  in a file-system-like name space.
</p>

<h3><a name="driveroperations">2.10.2 Driver Operations</a></h3>
<ul>
  <li><a href="#drvrfcntlops">2.10.2.1 <code>fcntl.h</code></a></li>
  <li><a href="#drvrunistdops">2.10.2.2 <code>unistd.h</code></a></li>
  <li><a href="#drvrioctlops">2.10.2.3 <code>sys/ioctl.h</code></a></li>
  <li><a href="#drvrpollops">2.10.2.4 <code>poll.h</code></a></li>
  <li><a href="#drvselectops">2.10.2.5 <code>sys/select.h</code></a></li>
</ul>

<h4><a name="drvrfcntlops">2.10.2.1 fcntl.h</a></h4>

<ul><pre>
  #include &lt;fcntl.h&gt;
  int creat(const char *path, mode_t mode);
  int open(const char *path, int oflag, ...);
  int fcntl(int fd, int cmd, ...);
</pre></ul>

<h4><a name="drvrunistdops">2.10.2.2 unistd.h</a></h4>

<ul><pre>
  #include &lt;unistd.h&gt;
  int     close(int fd);
  int     dup(int fd);
  int     dup2(int fd1, int fd2);
  off_t   lseek(int fd, off_t offset, int whence);
  ssize_t pread(int fd, void *buf, size_t nbytes, off_t offset);
  ssize_t pwrite(int fd, const void *buf, size_t nbytes, off_t offset);
  ssize_t read(int fd, void *buf, size_t nbytes);
  int     unlink(const char *path);
  ssize_t write(int fd, const void *buf, size_t nbytes);
</pre></ul>

<h4><a name="drvrioctlops">2.10.2.3 sys/ioctl.h</a></h4>

<ul><pre>
  #include &lt;sys/ioctl.h&gt;
  #ifdef CONFIG_LIBC_IOCTL_VARIADIC
  int ioctl(int fd, int req, ...);
  #else
  int ioctl(int fd, int req, unsigned long arg);
  #endif
</pre></ul>

<h4><a name="drvrpollops">2.10.2.4 poll.h</a></h4>

<h5><a name="poll">2.10.2.4.1 poll</a></H5>
<p>
  <b>Function Prototype:</b>
</p>
<pre>
  #include &lt;poll.h&gt;
  int poll(struct pollfd *fds, nfds_t nfds, int timeout);
</pre>
<p>
  <b>Description:</b>
   <code>poll()</code> waits for one of a set of file descriptors to become ready to
   perform I/O.  If none of the events requested (and no error) has
   occurred for any of  the  file  descriptors,  then  <code>poll()</code> blocks until
   one of the events occurs.
</p>
<p>
  <b>Configuration Settings</b>.
  In order to use the select with TCP/IP sockets test, you must have the following things
  selected in your NuttX configuration file:
</p>
<ul>
  <li><code>CONFIG_NET</code> Defined for general network support</li>
  <li><code>CONFIG_NET_TCP</code> Defined for TCP/IP support</li>
</ul>
<p>
  In order to for select to work with incoming connections, you must also select:
</p>
<ul>
  <li><code>CONFIG_NET_TCPBACKLOG</code>
  Incoming connections pend in a backlog until <code>accept()</code> is called.
  The size of the backlog is selected when <code>listen()</code> is called.</li>
</ul>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>fds</code>. List of structures describing file descriptors to be monitored.</li>
  <li><code>nfds</code>. The number of entries in the list.</li>
  <li><code>timeout</code>. Specifies an upper limit on the time for which <code>poll()</code> will
    block in milliseconds.  A negative value of <code>timeout</code> means an infinite
    timeout.</li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<p>
  On success, the number of structures that have nonzero <code>revents</code> fields.
  A value of 0 indicates that the call timed out and no file descriptors were ready.
  On error, -1 is returned, and <code>errno</code> is set appropriately:
</p>
<ul>
  <li><code>EBADF</code>. An invalid file descriptor was given in one of the sets.</li>
  <li><code>EFAULT</code>. The fds address is invalid</li>
  <li><code>EINTR</code>. A signal occurred before any requested event.</li>
  <li><code>EINVAL</code>. The nfds value exceeds a system limit.</li>
  <li><code>ENOMEM</code>. There was no space to allocate internal data structures.</li>
  <li><code>ENOSYS</code>. One or more of the drivers supporting the file descriptor does not support the poll method.</li>
</ul>

<h4><a name="drvselectops">2.10.2.5 sys/select.h</a></h4>

<h5><a name="select">2.10.2.5.1 select</a></H5>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/select.h&gt;
int select(int nfds, FAR fd_set *readfds, FAR fd_set *writefds,
           FAR fd_set *exceptfds, FAR struct timeval *timeout);
</pre></ul>
<p>
  <b>Description:</b>
  <code>select()</code> allows a program to monitor multiple file descriptors, waiting
  until one or more of the file descriptors become &quot;ready&quot; for some class
  of I/O operation (e.g., input possible).  A file descriptor is
  considered  ready if it is possible to perform the corresponding I/O
  operation (e.g., read(2)) without blocking.
</p>
<p>
  <b>NOTE:</b> <a href="#poll"><code>poll()</code></a> is the fundamental API for performing such monitoring
  operation under NuttX.  <code>select()</code> is provided for compatibility and
  is simply a layer of added logic on top of <code>poll()</code>.  As such, <code>select()</code>
  is more wasteful of resources and <a href="#poll"><code>poll()</code></a> is the recommended API to be
  used.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>nfds</code>. the maximum file descriptor number (+1) of any descriptor in any of the three sets.</li>
  <li><code>readfds</code>. the set of descriptions to monitor for read-ready events</li>
  <li><code>writefds</code>. the set of descriptions to monitor for write-ready events</li>
  <li><code>exceptfds</code>. the set of descriptions to monitor for error events</li>
  <li><code>timeout</code>. Return at this time if none of these events of interest occur.</li>
</ul>
<p>
  <b>Returned Value:</b>
</p>
<ul>
  <li><code>0:</code> Timer expired</li>
  <li><code>&gt;0:</code> The number of bits set in the three sets of descriptors</li>
  <li><code>-1:</code> An error occurred (<code>errno</code> will be set appropriately,
    see <a href="#poll"><code>poll()</code></a>).</li>
</ul>

<h3><a name="directoryoperations">2.10.3 Directory Operations</a></h3>
<a name="dirdirentops">
<ul><pre>
#include &lt;dirent.h&gt;

int        closedir(DIR *dirp);
FAR DIR   *opendir(const char *path);
FAR struct dirent *readdir(FAR DIR *dirp);
int        readdir_r(FAR DIR *dirp, FAR struct dirent *entry, FAR struct dirent **result);
void       rewinddir(FAR DIR *dirp);
void       seekdir(FAR DIR *dirp, int loc);
int        telldir(FAR DIR *dirp);
</pre></ul>
</a>

<h3><a name="dirunistdops">2.10.4 UNIX Standard Operations</a></h3>
<ul><pre>
#include &lt;unistd.h&gt;

/* Task Control Interfaces */

pid_t   vfork(void);
pid_t   getpid(void);
void    _exit(int status) noreturn_function;
unsigned int sleep(unsigned int seconds);
void    usleep(unsigned long usec);
int     pause(void);

/* File descriptor operations */

int     close(int fd);
int     dup(int fd);
int     dup2(int fd1, int fd2);
int     fsync(int fd);
off_t   lseek(int fd, off_t offset, int whence);
ssize_t read(int fd, FAR void *buf, size_t nbytes);
ssize_t write(int fd, FAR const void *buf, size_t nbytes);
ssize_t pread(int fd, FAR void *buf, size_t nbytes, off_t offset);
ssize_t pwrite(int fd, FAR const void *buf, size_t nbytes, off_t offset);

/* Check if a file descriptor corresponds to a terminal I/O file */

int     isatty(int fd);

/* Memory management */

#if defined(CONFIG_ARCH_ADDRENV) && defined(CONFIG_MM_PGALLOC) && \
    defined(CONFIG_ARCH_USE_MMU)
FAR void *sbrk(intptr_t incr);
#endif

/* Special devices */

int     pipe(int fd[2]);

/* Working directory operations */

int     chdir(FAR const char *path);
FAR char *getcwd(FAR char *buf, size_t size);

/* File path operations */

int     access(FAR const char *path, int amode);
int     rmdir(FAR const char *pathname);
int     unlink(FAR const char *pathname);

#ifdef CONFIG_PSEUDOFS_SOFTLINKS
int     link(FAR const char *path1, FAR const char *path2);
ssize_t readlink(FAR const char *path, FAR char *buf, size_t bufsize);
#endif

/* Execution of programs from files */

#ifdef CONFIG_LIBC_EXECFUNCS
int     execl(FAR const char *path, ...);
int     execv(FAR const char *path, FAR char *const argv[]);
#endif

/* Networking */

#ifdef CONFIG_NET
int     gethostname(FAR char *name, size_t size);
int     sethostname(FAR const char *name, size_t size);
#endif

/* Other */

int     getopt(int argc, FAR char *const argv[], FAR const char *optstring);

</pre></ul>
</a>

<h3><a name="standardio">2.10.5 Standard I/O</a></h3>
<ul><pre>
#include &lt;stdio.h&gt;

/* Operations on streams (FILE) */

void   clearerr(register FILE *stream);
int    fclose(FILE *stream);
int    fflush(FILE *stream);
int    feof(FILE *stream);
int    ferror(FILE *stream);
int    fileno(FAR FILE *stream);
int    fgetc(FILE *stream);
int    fgetpos(FILE *stream, fpos_t *pos);
char  *fgets(char *s, int n, FILE *stream);
FILE  *fopen(const char *path, const char *type);
int    fprintf(FILE *stream, const char *format, ...);
int    fputc(int c, FILE *stream);
int    fputs(const char *s, FILE *stream);
size_t fread(void *ptr, size_t size, size_t n_items, FILE *stream);
FAR FILE *freopen(FAR const char *path, FAR const char *mode,
         FAR FILE *stream);
int    fseek(FILE *stream, long int offset, int whence);
int    fsetpos(FILE *stream, fpos_t *pos);
long   ftell(FILE *stream);
size_t fwrite(const void *ptr, size_t size, size_t n_items, FILE *stream);
FAR char *gets(char *s);
FAR char *gets_s(FAR char *s, rsize_t n);
void   setbuf(FAR FILE *stream, FAR char *buf);
int    setvbuf(FAR FILE *stream, FAR char *buffer, int mode, size_t size);
int    ungetc(int c, FAR FILE *stream);

/* Operations on the stdout stream, buffers, paths, and the whole printf-family *    /

int    printf(const char *format, ...);
int    puts(const char *s);
int    rename(const char *source, const char *target);
int    sprintf(char *dest, const char *format, ...);
int    asprintf (FAR char **ptr, FAR const char *fmt, ...);
int    snprintf(FAR char *buf, size_t size, const char *format, ...);
int    sscanf(const char *buf, const char *fmt, ...);
void   perror(FAR const char *s);

int    vprintf(const char *s, va_list ap);
int    vfprintf(FILE *stream, const char *s, va_list ap);
int    vsprintf(char *buf, const char *s, va_list ap);
int    vasprintf(FAR char **ptr, const char *fmt, va_list ap);
int    vsnprintf(FAR char *buf, size_t size, const char *format, va_list ap);
int    vsscanf(char *buf, const char *s, va_list ap);

/* Operations on file descriptors including:
 *
 * POSIX-like File System Interfaces (fdopen), and
 * Extensions from the Open Group Technical Standard, 2006, Extended API Set
 *   Part 1 (dprintf and vdprintf)
 */

FAR FILE *fdopen(int fd, FAR const char *type);
int    dprintf(int fd, FAR const char *fmt, ...);
int    vdprintf(int fd, FAR const char *fmt, va_list ap);

/* Operations on paths */

FAR char *tmpnam(FAR char *s);
FAR char *tempnam(FAR const char *dir, FAR const char *pfx);
int       remove(FAR const char *path);

#include &lt;sys/stat.h&gt;

int mkdir(FAR const char *pathname, mode_t mode);
int mkfifo(FAR const char *pathname, mode_t mode);
int stat(const char *path, FAR struct stat *buf);
int fstat(int fd, FAR struct stat *buf);

#include &lt;sys/statfs.h&gt;

int statfs(FAR const char *path, FAR struct statfs *buf);
int fstatfs(int fd, FAR struct statfs *buf);
</pre></ul>

<h3><a name="standardlib">2.10.6 Standard Library</a></h3>
<p>
  <code>stdlib.h</code> generally addresses other operating system interfaces.
  However, the following may also be considered as file system interfaces:
</p>
<ul><pre>
#include &lt;stdlib.h&gt;

int mktemp(FAR char *template);
int mkstemp(FAR char *template);
</pre></ul>

<h3><a name="aio">2.10.7 Asynchronous I/O</a></h3>
<ul><pre>
#include &lt;aio.h&gt;

int aio_cancel(int, FAR struct aiocb *aiocbp);
int aio_error(FAR const struct aiocb *aiocbp);
int aio_fsync(int, FAR struct aiocb *aiocbp);
int aio_read(FAR struct aiocb *aiocbp);
ssize_t aio_return(FAR struct aiocb *aiocbp);
int aio_suspend(FAR const struct aiocb *const list[], int nent,
                FAR const struct timespec *timeout);
int aio_write(FAR struct aiocb *aiocbp);
int lio_listio(int mode, FAR struct aiocb *const list[], int nent,
               FAR struct sigevent *sig);
</pre></ul>

<h3><a name="stdstrings">2.10.8 Standard String Operations</a></h3>
<ul><pre>
#include &lt;string.h&gt;

char  *strchr(const char *s, int c);
FAR char *strdup(const char *s);
const char *strerror(int);
size_t strlen(const char *);
size_t strnlen(const char *, size_t);
char  *strcat(char *, const char *);
char  *strncat(char *, const char *, size_t);
int    strcmp(const char *, const char *);
int    strncmp(const char *, const char *, size_t);
int    strcasecmp(const char *, const char *);
int    strncasecmp(const char *, const char *, size_t);
char  *strcpy(char *dest, const char *src);
char  *strncpy(char *, const char *, size_t);
char  *strpbrk(const char *, const char *);
char  *strchr(const char *, int);
char  *strrchr(const char *, int);
size_t strspn(const char *, const char *);
size_t strcspn(const char *, const char *);
char  *strstr(const char *, const char *);
char  *strtok(char *, const char *);
char  *strtok_r(char *, const char *, char **);

void  *memset(void *s, int c, size_t n);
void  *memcpy(void *dest, const void *src, size_t n);
int    memcmp(const void *s1, const void *s2, size_t n);
void  *memmove(void *dest, const void *src, size_t count);

#include &lt;strings.h&gt;

#define bcmp(b1,b2,len)  memcmp(b1,b2,(size_t)len)
#define bcopy(b1,b2,len) (void)memmove(b2,b1,len)
#define bzero(s,n)       (void)memset(s,0,n)
#define index(s,c)       strchr(s,c)
#define rindex(s,c)      strrchr(s,c)

int    ffs(int j);
int    strcasecmp(const char *, const char *);
int    strncasecmp(const char *, const char *, size_t);
</pre></ul>

<h3><a name="PipesNFifos">2.10.9 Pipes and FIFOs</a></h3>

<h3>2.10.9.1 <a name="pipe"><code>pipe</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;unistd.h&gt;
int pipe(int fd[2]);
</pre></ul>
<p>
  <b>Description:</b>
  <ul>
    <p>
      <code>pipe()</code> creates a pair of file descriptors, pointing to a pipe inode, and
      places them in the array pointed to by <code>fd</code>.
      <code>fd[0]</code> is for reading, <code>fd[1]</code> is for writing.
    </p>
  </ul>
</p>
<p>
  <b>Input Parameters:</b>
  <ul>
    <li><code>fd[2]</code>.  The user provided array in which to catch the pipe file descriptors.</li>
  </ul>
</p>
</p>
<p>
  <b>Returned Value:</b>
  <ul>
    <p>
      0 is returned on success; otherwise, -1 is returned with <code>errno</code> set appropriately.
    </p>
  </ul>
</p>

<h3>2.10.9.2 <a name="mkfifo"><code>mkfifo</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/stat.h&gt;
int mkfifo(FAR const char *pathname, mode_t mode);
</pre></ul>
<p>
  <b>Description:</b>
  <ul>
    <p>
      <code>mkfifo()</code> makes a FIFO device driver file with name <code>pathname</code>.
      Unlike Linux, a NuttX FIFO is not a special file type but simply a device driver instance.
      <code>mode</code> specifies the FIFO's permissions (but is ignored in the current implementation).
    </p>
    <p>
      Once the FIFO has been created by <code>mkfifo()</code>, any thread can open it for
      reading or writing, in the same way as an ordinary file.
      However, it must have been opened from both reading and writing before input or output can be performed.
      This FIFO implementation will block all attempts to open a FIFO read-only until at least one thread has opened the FIFO for  writing.
    </p>
    <p>
      If all threads that write to the FIFO have closed, subsequent calls to <code>read()</code> on the FIFO will return 0 (end-of-file).
    </p>
  </ul>
</p>
<p>
  <b>Input Parameters:</b>
  <ul>
    <li><code>pathname</code>.
      The full path to the FIFO instance to attach to or to create (if not already created).
    </li>
    <li><code>mode</code>.
      Ignored for now
    </li>
  </ul>
</p>
<p>
  <b>Returned Value:</b>
  <ul>
    <p>
      0 is returned on success; otherwise, -1 is returned with <code>errno</code> set appropriately.
    </p>
  </ul>
</p>

<h3><a name="mmapxip">2.10.10 <code>mmap()</code> and eXecute In Place (XIP)</a></h3>
<p>
  NuttX operates in a flat open address space and is focused on MCUs that do
  support Memory Management Units (MMUs).  Therefore, NuttX generally does not
  require <code>mmap()</code> functionality and the MCUs generally cannot support true
  memory-mapped files.
</p>
<p>
  However, memory mapping of files is the mechanism used by NXFLAT, the NuttX
  tiny binary format, to get files into memory in order to execute them.
  <code>mmap()</code> support is therefore required to support NXFLAT.
  There are two conditions where <code>mmap()</code> can be supported:
</p>
<ol type="1">
  <li>
    <p>
      <code>mmap()</code> can be used to support <i>eXecute In Place</i> (XIP) on random access media
       under the following very restrictive conditions:
    </p>
    <ol type="a">
      <li>
        <p>
          The file-system supports the <code>FIOC_MMAP</code> ioctl command.
          Any file system that maps files contiguously on the media should support this
          <code>ioctl</code> command.
          By comparison, most file system scatter files over the media in non-contiguous
          sectors.  As of this writing, ROMFS is the only file system that meets this requirement.
        </p>
      </li>
      <li>
        <p>
          The underlying block driver supports the <code>BIOC_XIPBASE</code> <code>ioctl</code> command
          that maps the underlying media to a randomly accessible address.
           At present, only the RAM/ROM disk driver does this.
        </p>
      </li>
    </ol>
    <p>
       Some limitations of this approach are as follows:
    <p>
    <ol type="a">
      <li>
        <p>
          Since no real mapping occurs, all of the file contents are &quot;mapped&quot; into memory.
        </p>
      </li>
      <li>
        <p>
          All mapped files are read-only.
        </p>
      </li>
      <li>
        <p>
          There are no access privileges.
        </p>
      </li>
    </ol>
  </li>
  <li>
    <p>
      If <code>CONFIG_FS_RAMMAP</code> is defined in the configuration, then <code>mmap()</code> will
      support simulation of memory mapped files by copying files whole into RAM.
      These copied files have some of the properties of standard memory mapped files.
      There are many, many exceptions exceptions, however.
      Some of these include:
    </p>
    <ol type="a">
      <li>
        <p>
          The goal is to have a single region of memory that represents a single
          file and can be shared by many threads.  That is, given a filename a
          thread should be able to open the file, get a file descriptor, and
          call <code>mmap()</code> to get a memory region.  Different file descriptors opened
          with the same file path should get the same memory region when mapped.
        </p>
        <p>
          The limitation in the current design is that there is insufficient
          knowledge to know that these different file descriptors correspond to
          the same file.  So, for the time being, a new memory region is created
          each time that <code>rammmap()</code> is called. Not very useful!
        </p>
      </li>
      <li>
        <p>
          The entire mapped portion of the file must be present in memory.
          Since it is assumed that the MCU does not have an MMU, on-demanding
          paging in of file blocks cannot be supported. Since the while mapped
          portion of the file must be present in memory, there are limitations
          in the size of files that may be memory mapped (especially on MCUs
          with no significant RAM resources).
        </p>
      </li>
      <li>
        <p>
          All mapped files are read-only.  You can write to the in-memory image,
          but the file contents will not change.
        </p>
      </li>
      <li>
        <p>
          There are no access privileges.
        </p>
      </li>
      <li>
        <p>
          Since there are no processes in NuttX, all <code>mmap()</code> and <code>munmap()</code>
          operations have immediate, global effects.  Under Linux, for example,
          <code>munmap()</code> would eliminate only the mapping with a process; the mappings
          to the same file in other processes would not be effected.
        </p>
      </li>
      <li>
        <p>
          Like true mapped file, the region will persist after closing the file
          descriptor.  However, at present, these ram copied file regions are
          <i>not</i> automatically &quot;unmapped&quot; (i.e., freed) when a thread is terminated.
          This is primarily because it is not possible to know how many users
          of the mapped region there are and, therefore, when would be the
          appropriate time to free the region (other than when munmap is called).
        </p>
        <p>
          NOTE: Note, if the design limitation of a) were solved, then it would be
          easy to solve exception d) as well.
        </p>
      </li>
    </ol>
  </li>
</ol>

<h3><a name="mmap">2.10.11.1 <code>mmap</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/mman.h&gt;
FAR void *mmap(FAR void *start, size_t length, int prot, int flags, int fd, off_t offset);
</pre></ul>
<p>
  <b>Description:</b>
  <ul>
    Provides minimal <code>mmap()</code> as needed to support eXecute In Place (XIP)
    operation (as described above).
  </ul>
</p>
<p>
  <b>Input Parameters:</b>
  <ul>
    <li>
      <code>start</code>
        A hint at where to map the memory -- ignored.
        The address of the underlying media is fixed and cannot be re-mapped without MMU support.
    </li>
    <li>
      <code>length</code>
        The length of the mapping -- ignored.
        The entire underlying media is always accessible.
    </li>
    <li>
      <code>prot</code>
       See the <code>PROT_*</code> definitions in <code>sys/mman.h</code>.
       <ul>
         <li>
           <code>PROT_NONE</code> - Will cause an error.
         </li>
         <li>
           <code>PROT_READ</code> - <code>PROT_WRITE</code> and <code>PROT_EXEC</code> also assumed.
         </li>
         <li>
           <code>PROT_WRITE</code> - <code>PROT_READ</code> and <code>PROT_EXEC</code> also assumed.
         </li>
         <li>
           <code>PROT_EXEC</code> - <code>PROT_READ</code> and <code>PROT_WRITE</code> also assumed.
         </li>
       </ul>
    </li>
    <li>
      <code>flags</code>
      See the <code>MAP_*</code> definitions in <code>sys/mman.h</code>.
       <ul>
         <li>
           <code>MAP_SHARED</code> - Required
         </li>
         <li>
           <code>MAP_PRIVATE</code> - Will cause an error
         </li>
         <li>
           <code>MAP_FIXED</code> - Will cause an error
         </li>
         <li>
           <code>MAP_FILE</code> - Ignored
         </li>
         <li>
           <code>MAP_ANONYMOUS</code> - Will cause an error
         </li>
         <li>
           <code>MAP_ANON</code> - Will cause an error
         </li>
         <li>
           <code>MAP_GROWSDOWN</code> - Ignored
         </li>
         <li>
           <code>MAP_DENYWRITE</code> - Will cause an error
         </li>
         <li>
           <code>MAP_EXECUTABLE</code> - Ignored
         </li>
         <li>
           <code>MAP_LOCKED</code> - Ignored
         </li>
         <li>
           <code>MAP_NORESERVE</code> - Ignored
         </li>
         <li>
           <code>MAP_POPULATE</code> - Ignored
         </li>
         <li>
           <code>AP_NONBLOCK</code> - Ignored
         </li>
       </ul>
    </li>
    <li>
      <code>fd</code>
      file descriptor of the backing file -- required.
    </li>
    <li>
      <code>offset</code>
      The offset into the file to map.
    </li>
  </ul>
</p>
<p>
  <b>Returned Value:</b>
  <ul>
    <p>
      On success, <code>mmap()</code> returns a pointer to the mapped area.
      On error, the value <code>MAP_FAILED</code> is returned, and <code>errno</code> is set appropriately.
      <ul>
        <li><code>ENOSYS</code> -
          Returned if any of the unsupported <code>mmap()</code> features are attempted.
        </li>
        <li><code>EBADF</code> -
          <code>fd</code> is not a valid file descriptor.
        </li>
        <li><code>EINVAL</code> -
          Length is 0. flags contained neither <code>MAP_PRIVATE</code> or <code>MAP_SHARED</code>, or
          contained both of these values.
        </li>
        <li><code>ENODEV</code> -
          The underlying file-system of the specified file does not support memory mapping.
        </li>
      </ul>
    </p>
  </ul>
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Network"><h2>2.11 Network Interfaces</h2></a>
  </td>
  </tr>
</table>

<p>
  NuttX supports a BSD-compatible socket interface layer.
  These socket interface can be enabled by settings in the architecture <a href="NuttXConfigVariables.html">configuration file</a>.
  Those socket APIs are discussed in the following paragraphs.
</p>
<ul>
  <li><a href="#socket">2.11.1 socket</a></li>
  <li><a href="#bind">2.11.2 bind</a></li>
  <li><a href="#connect">2.11.3 connect</a></li>
  <li><a href="#listen">2.11.4 listen</a></li>
  <li><a href="#accept">2.11.5 accept</a></li>
  <li><a href="#send">2.11.6 send</a></li>
  <li><a href="#sendto">2.11.7 sendto</a></li>
  <li><a href="#recv">2.11.8 recv</a></li>
  <li><a href="#recvfrom">2.11.9 recvfrom</a></li>
  <li><a href="#setsockopt">2.11.10 setsockopt</a></li>
  <li><a href="#getsockopt">2.11.11 getsockopt</a></li>
</ul>

<h3><a name="socket">2.11.1 <code>socket</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int socket(int domain, int type, int protocol);
</pre></ul>
<p>
  <b>Description:</b>
   socket() creates an endpoint for communication and returns a descriptor.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>domain</code>: (see sys/socket.h)</li>
  <li><code>type</code>: (see sys/socket.h)</li>
  <li><code>protocol</code>: (see sys/socket.h)</li>
</ul>
<p>
  <b>Returned Value:</b>
  0 on success; -1 on error with <a href="#ErrnoAccess"><code>errno</code></a> set appropriately:
</p>
<ul>
  <li><code>EACCES</code>.
    Permission to create a socket of the specified type and/or protocol is denied.</li>
  <li><code>EAFNOSUPPORT</code>.
    The implementation does not support the specified address family.</li>
  <li><code>EINVAL</code>.
    Unknown protocol, or protocol family not available.</li>
  <li><code>EMFILE</code>.
    Process file table overflow.</li>
  <li><code>ENFILE</code>
    The system limit on the total number of open files has been reached.</li>
  <li><code>ENOBUFS</code> or <code>ENOMEM</code>.
    Insufficient memory is available. The socket cannot be created until sufficient resources are freed.</li>
  <li><code>EPROTONOSUPPORT</code>.
    The protocol type or the specified protocol is not supported within this domain.</li>
</ul>

<h3><a name="bind">2.11.2 <code>bind</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
</pre></ul>
<p>
  <b>Description:</b>
   <code>bind()</code> gives the socket sockfd the local address <code>addr</code>.
   <code>addr</code> is <code>addrlen</code> bytes long. Traditionally, this is called
   &quot;assigning a name to a socket.&quot; When a socket is created with <code>socket()</code>,
   it exists in a name space (address family) but has no name assigned.
<p>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>sockfd</code>: Socket descriptor from socket.</li>
  <li><code>addr</code>: Socket local address.</li>
  <li><code>addrlen</code>: Length of <code>addr</code>.</li>
</ul>
<p>
  <b>Returned Value:</b>
  0 on success; -1 on error with <a href="#ErrnoAccess"><code>errno</code></a> set appropriately:
</p>
<ul>
  <li><code>EACCES</code>
    The address is protected, and the user is not the superuser.</li>
  <li><code>EADDRINUSE</code>
    The given address is already in use.</li>
  <li><code>EBADF</code>
    <code>sockfd</code> is not a valid descriptor.</li>
  <li><code>EINVAL</code>
    The socket is already bound to an address.</li>
  <li><code>ENOTSOCK</code>
    <code>sockfd</code> is a descriptor for a file, not a socket.</li>
</ul>

<h3><a name="connect">2.11.3 <code>connect</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
</pre></ul>
<p>
  <b>Description:</b>
  <code>connect()</code> connects the socket referred to by the file descriptor
  <code>sockfd</code> to the address specified by <code>addr</code>.
  The <code>addrlen</code> argument specifies the size of <code>addr</code>.
  The format of the address in <code>addr</code> is determined by the address space
  of the socket sockfd.

  If the socket sockfd is of type SOCK_DGRAM then <code>addr</code> is the address
  to which datagrams are sent by default, and the only address from which
  datagrams are received. If the socket is of type SOCK_STREAM or
  SOCK_SEQPACKET, this call attempts to make a connection to the socket
  that is bound to the address specified by <code>addr</code>.

  Generally, connection-based protocol sockets may successfully <code>connect()</code>
  only once; connectionless protocol sockets may use <code>connect()</code> multiple
  times to change their association.  Connectionless sockets may dissolve
  the association by connecting to an address with the sa_family member of
  sockaddr set to AF_UNSPEC.
<p>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<p>
<ul>
  <li><code>sockfd</code>: Socket descriptor returned by <code>socket()</code></li>
  <li><code>addr</code>: Server address (form depends on type of socket)</li>
  <li><code>addrlen</code>: Length of actual <code>addr</code></li>
</ul>
<p>
  <b>Returned Value:</b>
  0 on success; -1 on error with <a href="#ErrnoAccess"><code>errno</code></a> set appropriately:
</p>
  <li><code>EACCES</code> or </code>EPERM</code>:
    The user tried to connect to a broadcast address without having the
    socket broadcast flag enabled or the connection request failed
    because of a local firewall rule.</li>
  <li><code>EADDRINUSE</code>
    Local address is already in use.</li>
  <li><code>EAFNOSUPPORT</code>
    The passed address didn't have the correct address family in its
    sa_family field.</li>
  <li><code>EAGAIN</code>
    No more free local ports or insufficient entries in the routing
    cache. For PF_INET.</li>
  <li><code>EALREADY</code>
    The socket is non-blocking and a previous connection attempt has
    not yet been completed.</li>
  <li><code>EBADF</code>
    The file descriptor is not a valid index in the descriptor table.</li>
  <li><code>ECONNREFUSED</code>
    No one listening on the remote address.</li>
  <li><code>EFAULT</code>
    The socket structure address is outside the user's address space.</li>
  <li><code>EINPROGRESS</code>
    The socket is non-blocking and the connection cannot be completed
    immediately.</li>
  <li><code>EINTR</code>
    The system call was interrupted by a signal that was caught.</li>
  <li><code>EISCONN</code>
    The socket is already connected.</li>
  <li><code>ENETUNREACH</code>
    Network is unreachable.</li>
  <li><code>ENOTSOCK</code>
    The file descriptor is not associated with a socket.</li>
  <li><code>ETIMEDOUT</code>
    Timeout while attempting connection. The server may be too busy
    to accept new connections.</li>
</ul>

<h3><a name="listen">2.11.4 listen</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int listen(int sockfd, int backlog);
</pre></ul>
<p>
  <b>Description:</b>
  To accept connections, a socket is first created with <code>socket()</code>, a
  willingness to accept incoming connections and a queue limit for incoming
  connections are specified with <code>listen()</code>, and then the connections are
  accepted with <code>accept()</code>. The <code>listen()</code> call applies only to sockets of
  type <code>SOCK_STREAM</code> or <code>SOCK_SEQPACKET</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>sockfd</code>: Socket descriptor of the bound socket.</li>
  <li><code>backlog</code>: The maximum length the queue of pending connections may grow.
    If a connection request arrives with the queue full, the client may receive an error
    with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission,
    the request may be ignored so that retries succeed.</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, zero is returned. On error, -1 is returned, and
  <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately.
</p>
<ul>
  <li><code>EADDRINUSE</code>: Another socket is already listening on the same port.</li>
  <li><code>EBADF</code>: The argument <code>sockfd</code> is not a valid descriptor.</li>
  <li><code>ENOTSOCK</code>: The argument <code>sockfd</code> is not a socket.</li>
  <li><code>EOPNOTSUPP</code>: The socket is not of a type that supports the listen operation.</li>
 </ul>

<h3><a name="accept">2.11.5 accept</a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>accept()</code> function is used with connection-based socket types
 (<code>SOCK_STREAM</code>, <code>SOCK_SEQPACKET</code> and <code>SOCK_RDM</code>).
  It extracts the first connection request on the queue of pending connections,
  creates a new connected socket with most of the same properties as <code>sockfd</code>,
  and allocates a new socket descriptor for the socket, which is returned. The
  newly created socket is no longer in the listening state. The original
  socket <code>sockfd</code> is unaffected by this call.  Per file descriptor flags
  are not inherited across an accept.
</p>
<p>
  The <code>sockfd</code> argument is a socket descriptor that has been created with
  <code>socket()</code>, bound to a local address with <code>bind()</code>, and is listening for
  connections after a call to <code>listen()</code>.
</p>
<p>
  On return, the <code>addr</code> structure is filled in with the address of the
  connecting entity. The <code>addrlen</code> argument initially contains the size
  of the structure pointed to by <code>addr</code>; on return it will contain the
  actual length of the address returned.
</p>
<p>
  If no pending connections are present on the queue, and the socket is
  not marked as non-blocking, accept blocks the caller until a connection
  is present. If the socket is marked non-blocking and no pending
  connections are present on the queue, accept returns <code>EAGAIN</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>sockfd</code>: Socket descriptor of the listening socket.</li>
  <li><code>addr</code>: Receives the address of the connecting client.</li>
  <li><code>addrlen</code>: Input: allocated size of <code>addr</code>, Return: returned size of <code>addr</code>.</li>
</ul>
<p>
  <b>Returned Value:</b>
  Returns -1 on error. If it succeeds, it returns a non-negative integer
  that is a descriptor for the accepted socket.
</p>
<ul>
  <li><code>EAGAIN</code> or <code>EWOULDBLOCK</code>:
    The socket is marked non-blocking and no connections are present to be accepted.</li>
  <li><code>EBADF</code>:
    The descriptor is invalid.</li>
  <li><code>ENOTSOCK</code>:
    The descriptor references a file, not a socket.</li>
  <li><code>EOPNOTSUPP</code>:
    The referenced socket is not of type <code>SOCK_STREAM</code>.</li>
  <li><code>EINTR</code>:
    The system call was interrupted by a signal that was caught before a valid connection arrived.</li>
  <li><code>ECONNABORTED</code>:
    A connection has been aborted.</li>
  <li><code>EINVAL</code>:
    Socket is not listening for connections.</li>
  <li><code>EMFILE</code>:
    The per-process limit of open file descriptors has been reached.</li>
  <li><code>ENFILE</code>:
    The system maximum for file descriptors has been reached.</li>
  <li><code>EFAULT</code>:
    The addr parameter is not in a writable part of the user address space.</li>
  <li><code>ENOBUFS</code> or <code>ENOMEM</code>:
    Not enough free memory.</li>
  <li><code>EPROTO</code>:
    Protocol error.</li>
  <li><code>EPERM</code>:
    Firewall rules forbid connection.</li>
</ul>

<h3><a name="send">2.11.6 <code>send</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
ssize_t send(int sockfd, const void *buf, size_t len, int flags);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>send()</code> call may be used only when the socket is in a connected state
  (so that the intended recipient is known).
  The only difference between <code>send()</code> and <code>write()</code> is the
  presence of <code>flags</code>.
  With <code>zero</code> flags parameter, <code>send()</code> is equivalent to
  <code>write()</code>. Also, <code>send(s,buf,len,flags)</code> is
  equivalent to <code>sendto(s,buf,len,flags,NULL,0)</code>.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>sockfd</code>: Socket descriptor of socket
  <li><code>buf</code>: Data to send
  <li><code>len</code>: Length of data to send
  <li><code>flags</code>: Send flags
</ul>
<p>
  <b>Returned Value:</b>
  See <a href="#sendto"><code>sendto()</code></a>.
</p>

<h3><a name="sendto">2.11.7 <code>sendto</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
 ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
               const struct sockaddr *to, socklen_t tolen);
</pre></ul>
<p>
  <b>Description:</b>
  If <code>sendto()</code> is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET)
  socket, the parameters to and tolen are ignored (and the error EISCONN
  may be returned when they are not NULL and 0), and the error ENOTCONN is
  returned when the socket was not actually connected.
<p>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>sockfd</code>: Socket descriptor of socket
  <li><code>buf</code>: Data to send
  <li><code>len</code>: Length of data to send
  <li><code>flags</code>: Send flags
  <li><code>to</code>: Address of recipient
  <li><code>tolen</code>: The length of the address structure
</ul>
<p>
  <b>Returned Value:</b>
  On success, returns the number of characters sent.  On  error, -1 is returned,
  and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
  <li><code>EAGAIN</code> or <code>EWOULDBLOCK</code>.
    The socket is marked non-blocking and the requested operation would block.
  <li><code>EBADF</code>.
    An invalid descriptor was specified.
  <li><code>ECONNRESET</code>.
    Connection reset by peer.
  <li><code>EDESTADDRREQ</code>.
    The socket is not connection-mode, and no peer address is set.
  <li><code>EFAULT</code>.
    An invalid user space address was specified for a parameter.
  <li><code>EINTR</code>.
    A signal occurred before any data was transmitted.
  <li><code>EINVAL</code>.
    Invalid argument passed.
  <li><code>EISCONN</code>.
    The connection-mode socket was connected already but a recipient
    was specified. (Now either this error is returned, or the recipient
    specification is ignored.)
  <li><code>EMSGSIZE</code>.
    The socket type requires that message be sent atomically, and the
    size of the message to be sent made this impossible.
  <li><code>ENOBUFS</code>.
    The output queue for a network interface was full. This generally
    indicates that the interface has stopped sending, but may be
    caused by transient congestion.
  <li><code>ENOMEM</code>.
    No memory available.
  <li><code>ENOTCONN</code>.
    The socket is not connected, and no target has been given.
  <li><code>ENOTSOCK</code>.
    The argument s is not a socket.
  <li><code>EOPNOTSUPP</code>.
    Some bit in the flags argument is inappropriate for the socket type.
  <li><code>EPIPE</code>.
    The local end has been shut down on a connection oriented socket.
    In this case the process will also receive a SIGPIPE unless
    MSG_NOSIGNAL is set.
</ul>

<h3><a name="recv">2.11.8 <code>recv</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
ssize_t recv(int sockfd, void *buf, size_t len, int flags);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>recv()</code> call is identical to
  <a href="#recvfrom"><code>recvfrom()</code></a> with a NULL
  <code>from</code> parameter.
<p>
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  </li>
  <li>sockfd</code>: Socket descriptor of socket </li>
  <li>buf</code>: Buffer to receive data </li>
  <li>len</code>: Length of buffer </li>
  <li>flags</code>: Receive flags </li>
</ul>
<p>
  <b>Returned Value:</b>
  See <a href="#recvfrom"><code>recvfrom()</code></a>.
</p>

<h3><a name="recvfrom">2.11.9 <code>recvfrom</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                 struct sockaddr *from, socklen_t *fromlen);
</pre></ul>
<p>
  <b>Description:</b>
  <code>recvfrom()</code> receives messages from a socket, and may be used to receive
  data on a socket whether or not it is connection-oriented.
</p>
<p>
  If <code>from</code> is not NULL, and the underlying protocol provides the source
  address, this source address is filled in. The argument <code>fromlen</code>
  initialized to the size of the buffer associated with <code>from</code>, and modified
  on return to indicate the actual size of the address stored there.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
    <li><code>sockfd</code>: Socket descriptor of socket.</li>
    <li><code>buf</code>:  Buffer to receive data.</li>
    <li><code>len</code>: Length of buffer.</li>
    <li><code>flags</code>: Receive flags.</li>
    <li><code>from</code>: Address of source.</li>
    <li><code>fromlen</code>: The length of the address structure.</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, returns the number of characters sent.
  If no data is available to be received and the peer has performed an orderly shutdown, recv() will return 0.
  Othwerwise, on errors, -1 is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
    <li><code>EAGAIN</code>.
      The socket is marked non-blocking and the receive operation would block,
      or a receive timeout had been set and the timeout expired before data
      was received.
    <li><code>EBADF</code>.
      The argument <code>sockfd</code> is an invalid descriptor.
    <li><code>ECONNREFUSED</code>.
      A remote host refused to allow the network connection (typically because
      it is not running the requested service).
    <li><code>EFAULT</code>.
      The receive buffer pointer(s) point outside the process's address space.
    <li><code>EINTR</code>.
      The receive was interrupted by delivery of a signal before any data were
      available.
    <li><code>EINVAL</code>.
      Invalid argument passed.
    <li><code>ENOMEM</code>.
      Could not allocate memory.
    <li><code>ENOTCONN</code>.
      The socket is associated with a connection-oriented protocol and has
      not been connected.
    <li><code>ENOTSOCK</code>.
      The argument <code>sockfd</code> does not refer to a socket.
</ul>

<h3><a name="setsockopt">2.11.10 <code>setsockopt</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int setsockopt(int sockfd, int level, int option,
               const void *value, socklen_t value_len);
</pre></ul>
<p>
  <b>Description:</b>
  <code>setsockopt()</code> sets the option specified by the <code>option</code> argument,
  at the protocol level specified by the <code>level</code> argument, to the value
  pointed to by the <code>value</code> argument for the socket associated with the
  file descriptor specified by the <code>sockfd</code> argument.
</p>
<p>
  The <code>level</code> argument specifies the protocol level of the option. To set
  options at the socket level, specify the level argument as SOL_SOCKET.
</p>
<p>
  See <code>sys/socket.h</code> for a complete list of values for the <code>option</code> argument.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>sockfd</code>: Socket descriptor of socket</li>
  <li><code>level</code>: Protocol level to set the option</li>
  <li><code>option</code>: identifies the option to set</li>
  <li><code>value</code>: Points to the argument value</li>
  <li><code>value_len</code>: The length of the argument value</li>
</ul>
<p>
  <b>Returned Value:</b>
  On success, returns the number of characters sent.
  On error, -1 is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
  <li><code>BADF</code>.
     The <code>sockfd</code> argument is not a valid socket descriptor.
  <li><code>DOM</code>.
     The send and receive timeout values are too big to fit into the
     timeout fields in the socket structure.
  <li><code>INVAL</code>.
     The specified option is invalid at the specified socket <code>level</code> or the
     socket has been shut down.
  <li><code>ISCONN</code>.
     The socket is already connected, and a specified option cannot be set
     while the socket is connected.
  <li><code>NOPROTOOPT</code>.
     The <code>option</code> is not supported by the protocol.
  <li><code>NOTSOCK</code>.
     The <code>sockfd</code> argument does not refer to a socket.
  <li><code>NOMEM</code>.
     There was insufficient memory available for the operation to complete.
  <li><code>NOBUFS</code>.
     Insufficient resources are available in the system to complete the call.
</ul>

<h3><a name="getsockopt">2.11.11 <code>getsockopt</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/socket.h&gt;
int getsockopt(int sockfd, int level, int option, void *value, socklen_t *value_len);
</pre></ul>
<p>
  <b>Description:</b>
  <code>getsockopt()</code> retrieve those value for the option specified by the
  <code>option</code> argument for the socket specified by the <code>sockfd</code> argument. If
  the size of the option value is greater than <code>value_len</code>, the value
  stored in the object pointed to by the <code>value</code> argument will be silently
  truncated. Otherwise, the length pointed to by the <code>value_len</code> argument
  will be modified to indicate the actual length of the<code>value</code>.
</p>
<p>
  The <code>level</code> argument specifies the protocol level of the option. To
  retrieve options at the socket level, specify the level argument as
  SOL_SOCKET.
</p>
<p>
  See <code>sys/socket.h</code> for a complete list of values for the <code>option</code> argument.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>sockfd</code>: Socket descriptor of socket
  <li><code>level</code>: Protocol level to set the option
  <li><code>option</code>: Identifies the option to get
  <li><code>value</code>:  Points to the argument value
  <li><code>value_len</code>: The length of the argument value
</ul>
<p>
  <b>Returned Value:</b>
  On success, returns the number of characters sent.
  On error, -1 is returned, and <a href="#ErrnoAccess"><code>errno</code></a> is set appropriately:
</p>
<ul>
  <li><code>BADF</code>.
    The <code>sockfd</code> argument is not a valid socket descriptor.</li>
  <li><code>INVAL</code>.
    The specified option is invalid at the specified socket <code>level</code> or the
    socket has been shutdown.</li>
  <li><code>NOPROTOOPT</code>.
    The <code>option</code> is not supported by the protocol.</li>
  <li><code>NOTSOCK</code>.
    The <code>sockfd</code> argument does not refer to a socket.</li>
  <li><code>NOBUFS</code>.
    Insufficient resources are available in the system to complete the call.</li>
</ul>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="SharedMemory"><h2>2.12 Shared Memory Interfaces</h2></a>
  </td>
  </tr>
</table>
<p>
  Shared memory interfaces are only available with the NuttX kernel build (<code>CONFIG_BUILD_KERNEL=y</code>).
  These interfaces support user memory regions that can be shared between multiple user processes.
  Shared memory interfaces:
</p>
<ul>
  <li><a href="#shmget">2.12.1 shmget</a></li>
  <li><a href="#shmat">2.12.2 shmat</a></li>
  <li><a href="#shmctl">2.12.3 shmctl</a></li>
  <li><a href="#shmdt">2.12.4 shmdt</a></li>
</ul>

<h3><a name="shmget">2.12.1 <code>shmget</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/shm.h&gt;
#include &lt;sys/ipc.h&gt;
int shmget(key_t key, size_t size, int shmflg);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>shmget()</code> function will return the shared memory identifier associated with <code>key</code>.
</p>
<p>
  A shared memory identifier, associated data structure, and shared memory segment of at least size bytes is created for <code>key</code> if one of the following is true:
</p>
<ul>
  <li>
    <p>
      The argument <code>key</code> is equal to <code>IPC_PRIVATE</code>.
    </p>
  </li>
  <li>
    <p>
      The argument <code>key</code> does not already have a shared memory identifier associated with it and <code>(shmflg & IPC_CREAT)</code> is non-zero.
    </p>
  </li>
</ul>
</p>
  Upon creation, the data structure associated with the new shared memory identifier will be initialized as follows:
</p>
<ul>
  <li>
    <p>
      The low-order nine bits of <code>shm_perm.mode</code> are set equal to the low-order nine bits of <code>shmflg</code>.
    </p>
  </li>
  <li>
    <p>
      The value of <code>shm_segsz</code> is set equal to the value of size.
    </p>
  </li>
  <li>
    <p>
      The values of <code>shm_lpid</code>, <code>shm_nattch</code>, <code>shm_atime</code>, and <code>shm_dtime</code> are set equal to 0.
    </p>
  </li>
  <li>
    <p>
      The value of <code>shm_ctime</code> is set equal to the current time.
    </p>
  </li>
</ul>
<p>
  When the shared memory segment is created, it will be initialized with all zero values.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li>
    <code>key</code>: The key that is used to access the unique shared memory identifier.
  </li>
  <li>
    <code>size</code>: The shared memory region that is created will be at least this size in bytes.
  </li>
  <li>
    <code>shmflg</code>:  See <code>IPC_*</code> definitions in <code>sys/ipc.h</code>.  Only the values <code>IPC_PRIVATE</code> or <code>IPC_CREAT</code> are supported.
  </li>
</ul>
<p>
  <b>Returned Value:</b>
  Upon successful completion, <code>shmget()</code> will return a non-negative integer, namely a shared memory identifier; otherwise, it will return -1 and set <code>errno</code> to indicate the error.
</p>
<ul>
  <li>
    <code>EACCES</code>.
      A shared memory identifier exists for key but operation permission as specified by the low-order nine bits of <code>shmflg</code> would not be granted.
  </li>
  <li>
    <code>EEXIST</code>.
      A shared memory identifier exists for the argument key but <code>(shmflg & IPC_CREAT) && (shmflg & IPC_EXCL)</code> are non-zero.
  </li>
  <li>
    <code>EINVAL</code>.
      A shared memory segment is to be created and the value of size is less than the system-imposed minimum or greater than the system-imposed maximum.
  </li>
  <li>
    <code>EINVAL</code>.
      No shared memory segment is to be created and a shared memory segment exists for key but the size of the segment associated with it is less than size and size is not 0.
  </li>
  <li>
    <code>ENOENT</code>.
      A shared memory identifier does not exist for the argument key and <code>(shmflg & IPC_CREAT)</code> is 0.
  </li>
  <li>
    <code>ENOMEM</code>.
      A shared memory identifier and associated shared memory segment will be created, but the amount of available physical memory is not sufficient to fill the request.
  </li>
  <li>
    <code>ENOSPC</code>.
      A shared memory identifier is to be created, but the system-imposed limit on the maximum number of allowed shared memory identifiers system-wide would be exceeded.
  </li>
</ul>
<p>
  <b>POSIX Deviations</b>
<p>
<ul>
  <li>
    <p>
      The values of <code>shm_perm.cuid</code>, <code>shm_perm.uid</code>, <code>shm_perm.cgid</code>, and <code>shm_perm.gid</code> should be set equal to the effective user ID and effective group ID, respectively, of the calling process.
      The NuttX <code>ipc_perm</code> structure, however, does not support these fields because user and group IDs are not yet supported by NuttX.
    </p>
  </li>
</ul>

<h3><a name="shmat">2.12.2 <code>shmat</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/shm.h&gt;
void *shmat(int shmid, FAR const void *shmaddr, int shmflg);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>shmat()</code> function attaches the shared memory segment associated with the shared memory identifier specified by <code>shmid</code> to the address space of the calling process. The segment is attached at the address specified by one of the following criteria:
</p>
<ul>
  <li>
    <p>
      If <code>shmaddr</code> is a null pointer, the segment is attached at the first available address as selected by the system.
    </p>
  </li>
  <li>
    <p>
      If <code>shmaddr</code> is not a null pointer and <code>(shmflg & SHM_RND)</code> is non-zero, the segment is attached at the address given by <code>(shmaddr - ((uintptr_t)shmaddr % SHMLBA))</code>.
    </p>
  </li>
  <li>
    <p>
      If <code>shmaddr</code> is not a null pointer and <code>(shmflg & SHM_RND)</code> is 0, the segment is attached at the address given by <code>shmaddr</code>.
    </p>
  </li>
  <li>
    <p>
      The segment is attached for reading if <code>(shmflg & SHM_RDONLY)</code> is non-zero and the calling process has read permission; otherwise, if it is 0 and the calling process has read and write permission, the segment is attached for reading and writing.
    </p>
  </li>
</ul>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>shmid</code>: Shared memory identifier</li>
  <li><code>smaddr</code>: Determines mapping of the shared memory region</li>
  <li><code>shmflg</code>: See <code>SHM_*</code> definitions in <code>include/sys/shm.h</code>.  Only <code>SHM_RDONLY</code> and <code>SHM_RND</code> are supported.</li>
</ul>
<p>
  <b>Returned Value:</b>
  Upon successful completion, <code>shmat()</code> will increment the value of <code>shm_nattch</code> in the data structure associated with the shared memory ID of the attached shared memory segment and return the segment's start address.

  Otherwise, the shared memory segment will not be attached, <code>shmat()</code> will return -1, and <code>errno</code> will be set to indicate the error.
</p>
<ul>
  <li>
    <code>EACCES</code>.
    Operation permission is denied to the calling process
  </li>
  <li>
    <code>EINVAL</code>.
    The value of <code>shmid</code> is not a valid shared memory identifier, the <code>shmaddr</code> is not a null pointer, and the value of <code>(shmaddr -((uintptr_t)shmaddr % SHMLBA))</code> is an illegal address for attaching shared memory; or the <code>shmaddr</code> is not a null pointer, <code>(shmflg & SHM_RND)</code> is 0, and the value of <code>shmaddr</code> is an illegal address for attaching shared memory.
  </li>
  <li>
    <code>EMFILE</code>.
    The number of shared memory segments attached to the calling process would exceed the system-imposed limit.
  </li>
  <li>
    <code>ENOMEM</code>.
    The available data space is not large enough to accommodate the shared memory segment.
  </li>
</ul>

<h3><a name="shmctl">2.12.3 <code>shmctl</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/shm.h&gt;
#include &lt;sys/ipc.h&gt;
int shmctl(int shmid, int cmd, FAR struct shmid_ds *buf);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>shmctl()</code> function provides a variety of shared memory control operations as specified by <code>cmd</code>. The following values for <code>cmd</code> are available:
</p>
<ul>
  <li>
    <p>
      <code>IPC_STAT</code>.
      Place the current value of each member of the <code>shmid_ds</code> data structure associated with <code>shmid</code> into the structure pointed to by <code>buf</code>.
    </p>
  </li>
  <li>
    <p>
      <code>IPC_SET</code>.
      Set the value of the <code>shm_perm.mode</code> member of the <code>shmid_ds</code> data structure associated with <code>shmid</code> to the corresponding value found in the structure pointed to by <code>buf</code>.
    </p>
  </li>
  <li>
    <p>
      <code>IPC_RMID</code>.
       Remove the shared memory identifier specified by <code>shmid</code> from the system and destroy the shared memory segment and <code>shmid_ds</code> data structure associated with it.
    </p>
  </li>
</ul>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>shmid</code>: Shared memory identifier</li>
  <li><code>cmd</code>: <code>shmctl()</code> command</li>
  <li><code>buf</code>: Data associated with the <code>shmctl()</code> command</li>
</ul>
<p>
  <b>Returned Value:</b>
  Upon successful completion, <code>shmctl()</code> will return 0; otherwise, it will return -1 and set <code>errno</code> to indicate the error.
</p>
<ul>
  <li>
    <code>EACCES</code>.
    The argument <code>cmd</code> is equal to <code>IPC_STAT</code> and the calling process does not have read permission.
  </li>
  <li>
    <code>EINVAL</code>.
    The value of <code>shmid</code> is not a valid shared memory identifier, or the value of <code>cmd </code>is not a valid command.
  </li>
  <li>
    <code>EPERM</code>.
    The argument <code>cmd</code> is equal to <code>IPC_RMID</code> or <code>IPC_SET</code> and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of <code>shm_perm.cuid</code> or <code>shm_perm.uid</code> in the data structure associated with <code>shmid</code>.
  </li>
  <li>
    <code>EOVERFLOW</code>.
    The <code>cmd</code> argument is <code>IPC_STAT</code> and the <code>gid</code> or <code>uid</code> value is too large to be stored in the structure pointed to by the <code>buf</code> argument.
  </li>
</ul>
<p>
  <b>POSIX Deviations</b>
<p>
<ul>
  <li>
    <code>IPC_SET</code>.
    Does not set the <code>shm_perm.uid</code> or <code>shm_perm.gid</code>members of the <code>shmid_ds</code> data structure associated with <code>shmid</code> because user and group IDs are not yet supported by NuttX
  </li>
  <li>
    <code>IPC_SET</code>.
    Does not restrict the operation to processes with appropriate privileges or matching user IDs in <code>shmid_ds</code> data structure associated with <code>shmid</code>.  Again because user IDs and user/group privileges are are not yet supported by NuttX
  </li>
  <li>
    <code>IPC_RMID</code>.
    Does not restrict the operation to processes with appropriate privileges or matching user IDs in <code>shmid_ds</code> data structure associated with <code>shmid</code>.  Again because user IDs and user/group privileges are are not yet supported by NuttX
  </li>
</ul>

<h3><a name="shmdt">2.12.4 <code>shmdt</code></a></h3>
<p>
  <b>Function Prototype:</b>
</p>
<ul><pre>
#include &lt;sys/shm.h&gt;
int shmdt(FAR const void *shmaddr);
</pre></ul>
<p>
  <b>Description:</b>
  The <code>shmdt()</code> function detaches the shared memory segment located at the address specified by <code>shmaddr</code> from the address space of the calling process.
</p>
<p>
  <b>Input Parameters:</b>
</p>
<ul>
  <li><code>shmid</code>: Shared memory identifier</li>
</ul>
<p>
  <b>Returned Value:</b>
  Upon successful completion, <code>shmdt()</code> will decrement the value of <code>shm_nattch</code> in the data structure associated with the shared memory ID of the attached shared memory segment and return 0.
</p>
<p>
  Otherwise, the shared memory segment will not be detached, <code>shmdt()</code> will return -1, and <code>errno</code> will be set to indicate the error.
</p>
<ul>
  <li>
    <code>EINVAL</code>.
    The value of <code>shmaddr</code> is not the data segment start address of a shared memory segment.
  </li>
</ul>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="Data_Structures"><h1>3.0 OS Data Structures</h1></a>
  </td>
  </tr>
</table>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="ScalarType"><h2>3.1 Scalar Types</h2></a>
  </td>
  </tr>
</table>

<p>
Many of the types used to communicate with NuttX are simple
scalar types. These types are used to provide architecture independence
of the OS from the application. The scalar types used at the NuttX
interface include:
<ul>
<li>pid_t
<li>size_t
<li>sigset_t
<li>time_t
</ul>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="HiddenStructures"><h2>3.2 Hidden Interface Structures</h2></a>
  </td>
  </tr>
</table>

<p>
Several of the types used to interface with NuttX are
structures that are intended to be hidden from the application.
From the standpoint of the application, these structures (and
structure pointers) should be treated as simple handles to reference
OS resources. These hidden structures include:
<ul>
<li>struct tcb_s
<li>mqd_t
<li>sem_t
<li>WDOG_ID
<li>pthread_key_t
</ul>
<p>
  In order to maintain portability, applications should not reference
  specific elements within these hidden structures. These hidden
  structures will not be described further in this user's manual.
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="ErrnoAccess"><h2>3.3 Access to the <code>errno</code> Variable</h2></a>
  </td>
  </tr>
</table>

<p>
  A pointer to the thread-specific <code>errno</code> value is available through a
  function call:
</p>
<p>
  <b>Function Prototype:</b>
<p>
<ul><pre>
#include &lt;errno.h&gt;
#define errno *get_errno_ptr()
int *get_errno_ptr(void);
</pre></ul>
<p>
  <b>Description</b>:
  <code>get_errno_ptr()</code> returns a pointer to the thread-specific <code>errno</code> value.
  Note that the symbol <code>errno</code> is defined to be <code>get_errno_ptr()</code> so that the usual
  access by referencing the symbol <code>errno</code> will work as expected.
</p>
<p>
  There is a unique, private <code>errno</code> value for each NuttX task.
  However, the implementation of <code>errno</code> differs somewhat from the use of
  <code>errno</code> in most multi-threaded process environments:
  In NuttX, each pthread will also have its own private copy of <code>errno</code> and the
  <code>errno</code> will not be shared between pthreads.
  This is, perhaps, non-standard but promotes better thread independence.
<p>
<b>Input Parameters</b>:  None
<p>
<b>Returned Values</b>:
<p>
<ul>
<li>A pointer to the thread-specific <code>errno</code> value.
</ul>
</p>

<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="UserStructures"><h2>3.4 User Interface Structures</h2></a>
  </td>
  </tr>
</table>

<H3>3.4.1 main_t</H3>
<p>
main_t defines the type of a task entry point. main_t is declared
in sys/types.h as:
<pre>
    typedef int (*main_t)(int argc, char *argv[]);
</pre>

<H3>3.4.2 struct sched_param</H3>

<p>
This structure is used to pass scheduling priorities to and from
NuttX;
<pre>
    struct sched_param
    {
      int sched_priority;
    };
</pre>

<H3>3.4.3 struct timespec</H3>

<p>
This structure is used to pass timing information between the
NuttX and a user application:
<pre>
    struct timespec
    {
      time_t tv_sec;  /* Seconds */
      long   tv_nsec; /* Nanoseconds */
    };
</pre>

<H3>3.4.4 struct mq_attr</H3>

<p>
This structure is used to communicate message queue attributes
between NuttX and a MoBY application:
<pre>
    struct mq_attr {
      size_t       mq_maxmsg;   /* Max number of messages in queue */
      size_t       mq_msgsize;  /* Max message size */
      unsigned     mq_flags;    /* Queue flags */
      size_t       mq_curmsgs;  /* Number of messages currently in queue */
    };
</pre>

<H3>3.4.5 struct sigaction</H3>

<p>
The following structure defines the action to take for given signal:
<pre>
    struct sigaction
    {
      union
      {
        void (*_sa_handler)(int);
        void (*_sa_sigaction)(int, siginfo_t *, void *);
      } sa_u;
      sigset_t           sa_mask;
      int                sa_flags;
    };
    #define sa_handler   sa_u._sa_handler
    #define sa_sigaction sa_u._sa_sigaction
</pre>

<H3>3.4.6 struct siginfo/siginfo_t</H3>

<p>
The following types is used to pass parameters to/from signal
handlers:
<pre>
    typedef struct siginfo
    {
      int          si_signo;
      int          si_code;
      union sigval si_value;
   } siginfo_t;
</pre>

<H3>3.4.7 union sigval</H3>

<p>
This defines the type of the struct siginfo si_value field and
is used to pass parameters with signals.
<pre>
    union sigval
    {
      int   sival_int;
      void *sival_ptr;
    };
</pre>

<H3>3.4.8 struct sigevent</H3>

<p>
The following is used to attach a signal to a message queue to
notify a task when a message is available on a queue.
<pre>
    struct sigevent
    {
      int          sigev_signo;
      union sigval sigev_value;
      int          sigev_notify;
    };
</pre>


<table width ="100%">
  <tr bgcolor="#e4e4e4">
  <td>
    <a name="index"><h1>Index</h1></a>
  </td>
  </tr>
</table>

<table width="100%">
<tr>
<td valign="top" width="34%">
  <li><a href="#accept">accept</a></li>
  <li><a href="#aio">aio.h</a></li>
  <li><a href="#aio">aio_cancel</a></li>
  <li><a href="#aio">aio_error</a></li>
  <li><a href="#aio">aio_fsync</a></li>
  <li><a href="#aio">aio_read</a></li>
  <li><a href="#aio">aio_return</a></li>
  <li><a href="#aio">aio_suspend</a></li>
  <li><a href="#aio">aio_write</a></li>
  <li><a href="#asctime">asctime</a></li>
  <li><a href="#asctimer">asctime_r</a></li>
  <li><a href="#atexit">atexit</a>
  <li><a href="#bind">bind</a></li>
  <li><a href="#mmapxip">BIOC_XIPBASE</a></li>
  <li><a href="#dirunistdops">chdir</a></li>
  <li><a href="#clockgetres">clock_getres</a></li>
  <li><a href="#clockgettime">clock_gettime</a></li>
  <li><a href="#ClocksNTimers">Clocks</a></li>
  <li><a href="#clocksettime">clock_settime</a></li>
  <li><a href="#drvrunistdops">close</a></li>
  <li><a href="#dirdirentops">closedir</a></li>
  <li><a href="#connect">connect</a></li>
  <li><a href="#drvrfcntlops">creat</a></li>
  <li><a href="#ctime">ctime</a></li>
  <li><a href="#ctimer">ctime_r</a></li>
  <li><a href="#Data_Structures">Data structures</a></li>
  <li><a href="#directoryoperations">Directory operations</a></li>
  <li><a href="#dirdirentops">dirent.h</a></li>
  <li><a href="#driveroperations">Driver operations</a></li>
  <li><a href="#drvrunistdops">dup</a></li>
  <li><a href="#drvrunistdops">dup2</a></li>
  <li><a href="#execl">execl</a></li>
  <li><a href="#mmapxip">eXecute In Place (XIP)</a></li>
  <li><a href="#exec">exec</a></li>
  <li><a href="#execv">execv</a></li>
  <li><a href="#exit">exit</a></li>
  <li><a href="#fatsupport">FAT File System Support</a></li>
  <li><a href="#standardio">fclose</a></li>
  <li><a href="#drvrfcntlops">fcntl</a></li>
  <li><a href="#drvrfcntlops">fcntl.h</a></li>
  <li><a href="#standardio">fdopen</a></li>
  <li><a href="#standardio">feof</a></li>
  <li><a href="#standardio">ferror</a></li>
  <li><a href="#FileSystem">File system, interfaces</a></li>
  <li><a href="#FileSystemOverview">File system, overview</a></li>
  <li><a href="#standardio">fflush</a></li>
  <li><a href="#standardio">fgetc</a></li>
  <li><a href="#standardio">fgetpos</a></li>
  <li><a href="#standardio">fgets</a></li>
  <li><a href="#mmapxip">FIOC_MMAP</a></li>
  <li><a href="#standardio">fopen</a></li>
  <li><a href="#standardio">fprintf</a></li>
  <li><a href="#standardio">fputc</a></li>
  <li><a href="#standardio">fputs</a></li>
  <li><a href="#standardio">fread</a></li>
  <li><a href="#standardio">fseek</a></li>
  <li><a href="#standardio">fsetpos</a></li>
  <li><a href="#standardio">fstat</a></li>
  <li><a href="#standardio">ftell</a></li>
  <li><a href="#standardio">fwrite</a></li>
  <li><a href="#dirunistdops">getcwd</a></li>
  <li><a href="#getpid">getpid</a></li>
  <li><a href="#standardio">gets</a></li>
  <li><a href="#getsockopt">getsockopt</a></li>
  <li><a href="#gmtime">gmtime</a></li>
  <li><a href="#gmtimer">gmtime_r</a></li>
  <li><a href="#Introduction">Introduction</a>
  <li><a href="#drvrioctlops">ioctl</a></li>
  <li><a href="#kill">kill</a></li>
  <li><a href="#aio">lio_listio</a></li>
  <li><a href="#listen">listen</a></li>
  <li><a href="#localtimer">localtime_r</a></li>
  <li><a href="#lockingvssignaling">Locking versus Signaling Semaphores</a></li>
  <li><a href="#drvrunistdops">lseek</a></li>
  <li><a href="#Message_Queue">Named Message Queue Interfaces</a>
  <li><a href="#standardio">mkdir</a></li>
  <li><a href="#mkfifo">mkfifo</a></li>
  <li><a href="#mktime">mktime</a></li>
  <li><a href="#mqclose">mq_close</a></li>
  <li><a href="#mqgetattr">mq_getattr</a></li>
  <li><a href="#mqnotify">mq_notify</a></li>
  <li><a href="#mqopen">mq_open</a></li>
  <li><a href="#mqreceive">mq_receive</a></li>
  <li><a href="#mqsend">mq_send</a></li>
  <li><a href="#mqsetattr">mq_setattr</a></li>
  <li><a href="#mqtimedreceive">mq_timedreceive</a></li>
  <li><a href="#mqtimedsend">mq_timedsend</a></li>
  <li><a href="#mqunlink">mq_unlink</a></li>
  <li><a href="#mmap">mmap</a></li>
  <li><a href="#Network">Network Interfaces</a></li>
</td>
<td valign="top" width="33%">
  <li><a href="#onexit">on_exit</a>
  <li><a href="#drvrfcntlops">open</a></li>
  <li><a href="#dirdirentops">opendir</a></li>
  <li><a href="#OS_Interfaces">OS Interfaces</a></li>
  <li><a href="#pause">pause</a></li>
  <li><a href="#pipe">pipe</a></li>
  <li><a href="#poll">poll</a></li>
  <li><a href="#drvrpollops">poll.h</a></li>
  <li><a href="#posix_spawn">posix_spawn</a></li>
  <li><a href="#posix_spawn_file_actions_addclose">posix_spawn_file_actions_addclose</a></li>
  <li><a href="#posix_spawn_file_actions_adddup2">posix_spawn_file_actions_adddup2</a></li>
  <li><a href="#posix_spawn_file_actions_addopen">posix_spawn_file_actions_addopen</a></li>
  <li><a href="#posix_spawn_file_actions_destroy">posix_spawn_file_actions_destroy</a></li>
  <li><a href="#posix_spawn_file_actions_init">posix_spawn_file_actions_init</a></li>
  <li><a href="#posix_spawnattr_init">posix_spawnattr_init</a></li>
  <li><a href="#posix_spawnattr_init">posix_spawnattr_destroy</a></li>
  <li><a href="#posix_spawnattr_getflags">posix_spawnattr_getflags</a></li>
  <li><a href="#posix_spawnattr_getschedparam">posix_spawnattr_getschedparam</a></li>
  <li><a href="#posix_spawnattr_getschedpolicy">posix_spawnattr_getschedpolicy</a></li>
  <li><a href="#posix_spawnattr_getsigmask">posix_spawnattr_getsigmask</a></li>
  <li><a href="#posix_spawnattr_setflags">posix_spawnattr_setflags</a></li>
  <li><a href="#posix_spawnattr_setschedparam">posix_spawnattr_setschedparam</a></li>
  <li><a href="#posix_spawnattr_setschedpolicy">posix_spawnattr_setschedpolicy</a></li>
  <li><a href="#posix_spawnattr_setsigmask">posix_spawnattr_setsigmask</a></li>
  <li><a href="#posix_spawn">posix_spawnp</a></li>
  <li><a href="#drvrunistdops">pread</a></li>
  <li><a href="#standardio">printf</a></li>
  <li><a href="#drvrunistdops">pwrite</a></li>
  <li><a href="#Pthread">Pthread Interfaces</a>
  <li><a href="#pthreadattrdestroy">pthread_attr_destroy</a></li>
  <li><a href="#pthreadattrgetinheritsched">pthread_attr_getinheritsched</a></li>
  <li><a href="#pthreadattrgetschedparam">pthread_attr_getschedparam</a></li>
  <li><a href="#pthreadattrgetschedpolicy">pthread_attr_getschedpolicy</a></li>
  <li><a href="#pthreadattrgetstacksize">pthread_attr_getstacksize</a></li>
  <li><a href="#pthreadattrinit">pthread_attr_init</a></li>
  <li><a href="#pthreadattrsetinheritsched">pthread_attr_setinheritsched</a></li>
  <li><a href="#pthreadattrsetschedparam">pthread_attr_setschedparam</a></li>
  <li><a href="#pthreadattrsetschedpolity">pthread_attr_setschedpolicy</a></li>
  <li><a href="#pthreadattrsetstacksize">pthread_attr_setstacksize</a></li>
  <li><a href="#pthreadbarrierattrinit">pthread_barrierattr_init</a></li>
  <li><a href="#pthreadbarrierattrdestroy">pthread_barrierattr_destroy</a></li>
  <li><a href="#pthreadbarrierattrgetpshared">pthread_barrierattr_getpshared</a></li>
  <li><a href="#pthreadbarrierattrsetpshared">pthread_barrierattr_setpshared</a></li>
  <li><a href="#pthreadbarrierdestroy">pthread_barrier_destroy</a></li>
  <li><a href="#pthreadbarrierinit">pthread_barrier_init</a></li>
  <li><a href="#pthreadbarrierwait">pthread_barrier_wait</a></li>
  <li><a href="#pthreadcancel">pthread_cancel</a></li>
  <li><a href="#pthreadconaddrinit">pthread_condattr_init</a></li>
  <li><a href="#pthreadcondbroadcast">pthread_cond_broadcast</a></li>
  <li><a href="#pthreadconddestroy">pthread_cond_destroy</a></li>
  <li><a href="#pthreadcondinit">pthread_cond_init</a></li>
  <li><a href="#pthreadcondsignal">pthread_cond_signal</a></li>
  <li><a href="#pthreadcondtimedwait">pthread_cond_timedwait</a></li>
  <li><a href="#pthreadcondwait">pthread_cond_wait</a></li>
  <li><a href="#pthreadcreate">pthread_create</a></li>
  <li><a href="#pthreaddetach">pthread_detach</a></li>
  <li><a href="#pthreadexit">pthread_exit</a></li>
  <li><a href="#pthreadgetschedparam">pthread_getschedparam</a></li>
  <li><a href="#pthreadgetspecific">pthread_getspecific</a></li>
  <li><a href="#Pthread"><i>pthreads</i></a> share some resources.
  <li><a href="#pthreadjoin">pthread_join</a></li>
  <li><a href="#pthreadkeycreate">pthread_key_create</a></li>
  <li><a href="#pthreadkeydelete">pthread_key_delete</a></li>
  <li><a href="#pthreadkill">pthread_kill</a></li>
  <li><a href="#pthreadmutexattrdestroy">pthread_mutexattr_destroy</a></li>
  <li><a href="#pthreadmutexattrgetprotocol">pthread_mutexattr_getprotocol</a></li>
  <li><a href="#pthreadmutexattrgetpshared">pthread_mutexattr_getpshared</a></li>
  <li><a href="#pthreadmutexattrgettype">pthread_mutexattr_gettype</a></li>
  <li><a href="#pthreadmutexattrinit">pthread_mutexattr_init</a></li>
  <li><a href="#pthreadmutexattrsetprotocol">pthread_mutexattr_setprotocol</a></li>
  <li><a href="#pthreadmutexattrsetpshared">pthread_mutexattr_setpshared</a></li>
  <li><a href="#pthreadmutexattrsettype">pthread_mutexattr_settype</a></li>
  <li><a href="#pthreadmutexdestrory">pthread_mutex_destroy</a></li>
  <li><a href="#pthreadmutexinit">pthread_mutex_init</a></li>
  <li><a href="#pthreadmutexlock">pthread_mutex_lock</a></li>
  <li><a href="#pthreadmutextimedlock">pthread_mutex_timedlock</a></li>
  <li><a href="#pthreadmutextrylock">pthread_mutex_trylock</a></li>
  <li><a href="#pthreadmutexunlock">pthread_mutex_unlock</a></li>
  <li><a href="#pthreadocndattrdestroy">pthread_condattr_destroy</a></li>
  <li><a href="#pthreadonce">pthread_once</a></li>
  <li><a href="#pthreadself">pthread_self</a></li>
  <li><a href="#pthreadsetcancelstate">pthread_setcancelstate</a></li>
  <li><a href="#pthreadsetcanceltype">pthread_setcanceltype</a></li>
  <li><a href="#pthreadsetschedparam">pthread_setschedparam</a></li>
  <li><a href="#pthreadsetspecific">pthread_setspecific</a></li>
  <li><a href="#pthreadsigmask">pthread_sigmask</a></li>
  <li><a href="#pthreadtestcancel">pthread_testcancel</a></li>
  <li><a href="#pthreadyield">pthread_yield</a></li>
  <li><a href="#standardio">puts</a></li>
  <li><a href="#mmapxip">RAM disk driver</a></li>
  <li><a href="#drvrunistdops">read</a></li>
</td>
<td valign="top">
  <li><a href="#dirdirentops">readdir</a></li>
  <li><a href="#dirdirentops">readdir_r</a></li>
  <li><a href="#recv">recv</a></li>
  <li><a href="#recvfrom">recvfrom</a></li>
  <li><a href="#standardio">rename</a></li>
  <li><a href="#standardio">rmdir</a></li>
  <li><a href="#dirdirentops">rewinddir</a></li>
  <li><a href="#mmapxip">ROM disk driver</a></li>
  <li><a href="#mmapxip">ROMFS</a></li>
  <li><a href="#schedgetparam">sched_getparam</a></li>
  <li><a href="#schedgetprioritymax">sched_get_priority_max</a></li>
  <li><a href="#schedgetprioritymin">sched_get_priority_min</a></li>
  <li><a href="#schedgetrrinterval">sched_get_rr_interval</a></li>
  <li><a href="#schedlockcount">sched_lockcount</a></li>
  <li><a href="#schedlock">sched_lock</a></li>
  <li><a href="#schedsetparam">sched_setparam</a></li>
  <li><a href="#schedsetscheduler">sched_setscheduler</a></li>
  <li><a href="#schedunlock">sched_unlock</a></li>
  <li><a href="#sched_yield">sched_yield</a></li>
  <li><a href="#select">select</a></li>
  <li><a href="#Semaphores">Counting Semaphore Interfaces</a>
  <li><a href="#semclose">sem_close</a></li>
  <li><a href="#semdestroy">sem_destroy</a></li>
  <li><a href="#semgetprotocol">sem_getprotocol</a></li>
  <li><a href="#semgetvalue">sem_getvalue</a></li>
  <li><a href="#seminit">sem_init</a></li>
  <li><a href="#semopen">sem_open</a></li>
  <li><a href="#sempost">sem_post</a></li>
  <li><a href="#semsetprotocol">sem_setprotocol</a></li>
  <li><a href="#semtrywait">sem_trywait</a></li>
  <li><a href="#semunlink">sem_unlink</a></li>
  <li><a href="#semwait">sem_wait</a></li>
  <li><a href="#setgetscheduler">sched_getscheduler</a></li>
  <li><a href="#dirdirentops">seekdir</a></li>
  <li><a href="#send">send</a></li>
  <li><a href="#sendto">sendto</a></li>
  <li><a href="#setsockopt">setsockopt</a></li>
  <li><a href="#sigaction">sigaction</a></li>
  <li><a href="#sigaddset">sigaddset</a></li>
  <li><a href="#sigdelset">sigdelset</a></li>
  <li><a href="#sigemptyset">sigemptyset</a></li>
  <li><a href="#sigfillset">sigfillset</a></li>
  <li><a href="#sigismember">sigismember</a></li>
  <li><a href="#Signals">Signal Interfaces</a>
  <li><a href="#sigpending">sigpending</a></li>
  <li><a href="#sigprocmask">sigprocmask</a></li>
  <li><a href="#sigqueue">sigqueue</a></li>
  <li><a href="#sigsuspend">sigsuspend</a></li>
  <li><a href="#sigtimedwait">sigtimedwait</a></li>
  <li><a href="#sigwaitinfo">sigwaitinfo</a></li>
  <li><a href="#socket">socket</a></li>
  <li><a href="#standardio">sprintf</a></li>
  <li><a href="#standardio">Standard I/O</a></li>
  <li><a href="#standardio">stat</a></li>
  <li><a href="#standardio">statfs</a></li>
  <li><a href="#standardio">stdio.h</a></li>
  <li><a href="#drvselectops">sys/select.h</a></li>
  <li><a href="#drvrioctlops">sys/ioctl.h</a></li>
  <li><a href="#taskactivate">task_activate</a></li>
  <li><a href="#Task_Control">Task Control Interfaces</a>
  <li><a href="#taskcreate">task_create</a></li>
  <li><a href="#taskdelete">task_delete</a></li>
  <li><a href="#taskinit">task_init</a></li>
  <li><a href="#taskrestart">task_restart</a></li>
  <li><a href="#Task_Schedule">Task Scheduling Interfaces</a>
  <li><a href="#tasksetcancelstate">task_setcancelstate</a></li>
  <li><a href="#tasksetcanceltype>task_setcanceltype</a></li>
  <li><a href="#task_spawn">task_spawn</a></li>
  <li><a href="#task_spawnattr_getstacksize">task_spawnattr_getstacksize</a></li>
  <li><a href="#task_spawnattr_setstacksize">task_spawnattr_setstacksize</a></li>
  <li><a href="#Task_Switch">Task Switching Interfaces</a>
  <li><a href="#tasktestcancel">task_testcancel</a></li>
  <li><a href="#dirdirentops">telldir</a></li>
  <li><a href="#timercreate">timer_create</a></li>
  <li><a href="#timerdelete">timer_delete</a></li>
  <li><a href="#timergetoverrun">timer_getoverrun</a></li>
  <li><a href="#timergettime">timer_gettime</a></li>
  <li><a href="#ClocksNTimers">Timers</a></li>
  <li><a href="#timersettime">timer_settime</a></li>
  <li><a href="#standardio">ungetc</a></li>
  <li><a href="#drvrunistdops">unistd.h</a>,
      <a href="#dirunistdops">unistd.h</a></li>
  <li><a href="#drvrunistdops">unlink</a></li>
  <li><a href="#vfork">vfork</a></li>
  <li><a href="#standardio">vfprintf</a></li>
  <li><a href="#standardio">vprintf</a></li>
  <li><a href="#standardio">vsprintf</a></li>
  <li><a href="#wait">wait</a></li>
  <li><a href="#waitid">waitid</a></li>
  <li><a href="#waitpid">waitpid</a>
  <li><a href="#drvrunistdops">write</a></li>
  <li><a href="#mmapxip">XIP</a></li>
</td>
</tr>
</table>

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